kernfs: fix potential NULL dereference in __kernfs_remove

In the Linux kernel, the following vulnerability has been resolved: kernfs: fix potential NULL dereference in __kernfs_remove When lockdep is enabled, lockdep_assert_held_write would cause potential NULL pointer dereference. Fix the following smatch warnings: fs/kernfs/dir.c:1353 __kernfs_remove() warn: variable dereferenced before check 'kn' (see line 1346)