f2fs: do sanity check on i_xattr_nid

f2fs: do sanity check on i_xattr_nid

This is a regression test:
1. create directory
2. add a new xattr entry to create xattr node
3. use inject.f2fs to inject nid of xattr node w/ ino in a file
4. check whether f2fs kernel module will detect and report such
   corruption in the file

Cc: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Chao Yu <chao@kernel.org>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>