Let the BPF verifier to recognize const char * arguments from LSM hooks
(and other BPF program types) as valid const string pointers that can be
passed to kfuncs expecting KF_ARG_PTR_TO_CONST_STR.

Previously, kfuncs with KF_ARG_PTR_TO_CONST_STR only accepted
PTR_TO_MAP_VALUE from readonly maps. This was limiting for LSM programs
that receive const char * arguments from hooks like sb_mount's dev_name.

Signed-off-by: Song Liu <[email protected]>

Add two new kfuncs to fs/bpf_fs_kfuncs.c that wrap kern_path() for use
by BPF LSM programs:

bpf_kern_path():
- Resolves a pathname string to a struct path
- Allocates memory for the path structure
- Returns NULL on error or if the path doesn't exist
- Marked with KF_ACQUIRE | KF_SLEEPABLE | KF_RET_NULL

bpf_path_put():
- Releases the path reference and frees the allocated memory
- Marked with KF_RELEASE to enforce acquire/release semantics

These kfuncs enable BPF LSM programs to resolve pathnames provided by
hook arguments (e.g., dev_name from sb_mount) and validate or inspect
the resolved paths. The verifier enforces proper resource management
through acquire/release tracking.

Example usage:
  struct path *p = bpf_kern_path("/etc/passwd", LOOKUP_FOLLOW);
  if (p) {
      // Use the path...
      bpf_path_put(p);  // Must release
  }

Signed-off-by: Song Liu <[email protected]>