fs/inode: add optional manual FD backtrace control via task group flag

When CONFIG_FS_BACKTRACE is enabled, collecting a stack trace for every
new file descriptor adds overhead to fast path operations like open(),
dup(), and socket().

This patch adds a new configuration option CONFIG_FS_BACKTRACE_DEFAULT.
When enabled (default behavior), the GROUP_FLAG_FD_BACKTRACE flag is
automatically set during group allocation, causing backtrace to be
captured for all tasks globally, preserving the original diagnostic
capability.

When disabled, backtrace capture is zero-cost by default and must be
explicitly enabled per task group using GROUP_FLAG_FD_BACKTRACE.

Signed-off-by: zhanxiaoqi <zhanxiaoqi@bytedance.com>

include/nuttx/sched.h

@@ -117,6 +117,7 @@
 #define GROUP_FLAG_PRIVILEGED      (1 << 1)                      /* Bit 1: Group is privileged */
 #define GROUP_FLAG_DELETED         (1 << 2)                      /* Bit 2: Group has been deleted but not yet freed */
 #define GROUP_FLAG_EXITING         (1 << 3)                      /* Bit 3: Group exit is in progress */
+#define GROUP_FLAG_FD_BACKTRACE    (1 << 4)                      /* Bit 4: Enable FD backtrace for the group */
                                                                  /* Bits 3-7: Available */
 
 /* Values for struct child_status_s ch_flags */