fs/inode: add dynamic FD backtrace threshold control

[xiaoqizhan]

When CONFIG_FS_BACKTRACE is enabled, collecting a stack trace for every new file descriptor adds avoidable overhead to tasks that use only a small number of FDs.

Add a dynamic threshold option so backtrace capture is enabled only after the per-task FD count reaches the configured limit.

Note: Please adhere to Contributing Guidelines.

Summary

This patch introduces a dynamic backtrace threshold control mechanism for file descriptors.

**Why the change is necessary:**
Currently, when `CONFIG_FS_BACKTRACE` is enabled, the system collects a stack backtrace for every newly allocated file descriptor. However, this imposes unnecessary and avoidable

performance overhead on tasks that only allocate a small number of file descriptors and never exceed typical limits or experience FD leaks.

**What it does and how:**
This change introduces `CONFIG_FS_BACKTRACE_DYNAMIC`. When enabled, it avoids capturing stack traces for normal, small-scale file descriptor allocations. The system tracks the number of

open file descriptors per task (fl_open_count) and only begins filling the backtrace array (f_backtrace) once the per-task file descriptor count reaches a configurable threshold
(CONFIG_FS_BACKTRACE_DEFAULT_THRESHOLD).

This feature significantly improves runtime performance for tasks with normal FD usage while still preserving the powerful diagnostic capabilities of `CONFIG_FS_BACKTRACE` for detecting

resource leaks when they actually occur.

Impact

* **Users:** Improves runtime performance and reduces system overhead by omitting backtrace collection for tasks that use few file descriptors.
* **Build process:** Adds new Kconfig options (`CONFIG_FS_BACKTRACE_DYNAMIC` and `CONFIG_FS_BACKTRACE_DEFAULT_THRESHOLD`).
* **Hardware:** None.
* **Documentation:** No significant impact, but the Kconfig help text has been updated to reflect the new feature.
* **Security:** None.
* **Compatibility:** Backward compatible. The dynamic feature defaults to `n`, preserving the original behavior of `CONFIG_FS_BACKTRACE` unless explicitly enabled.

Testing

Host Machine: Ubuntu Linux (x86_64)
Target: Verified on generic simulators and local boards (e.g. sim:nsh)

**Verification Steps:**
1. Built the system with `CONFIG_FS_BACKTRACE=y` and `CONFIG_FS_BACKTRACE_DYNAMIC=n` to ensure the original backtrace logic works without regression.
2. Built the system with `CONFIG_FS_BACKTRACE_DYNAMIC=y` and set a default threshold (e.g., 60).
3. Created an application that opens multiple files iteratively:
   * Verified that backtraces are **not** captured for the first 59 file descriptors.
   * Verified that once the threshold (60th FD) is reached, the `fl_bt_enabled` flag is set, and backtraces are successfully captured and stored for subsequent file descriptors.
4. Verified that file descriptors closed (`fl_open_count` decremented) do not cause the threshold trigger to oscillate incorrectly (once enabled, it stays enabled for that task's

fdlist).
5. Tested fdlist_copy (task creation) to ensure backtrace thresholds and states are correctly propagated/initialized for child tasks.