f2fs: do sanity check on mapping table

f2fs: do sanity check on mapping table

This is a regression test:
1. create a file
2. write file to create a direct node at special offset
3. use inject.f2fs to inject nid of direct node w/ ino of the inode
4. check whether f2fs kernel module will detect and report such
   corruption in the fil

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