fsck.f2fs: attempt to identify bad dentries

If a dentry has a bad ino (fsck_chk_nod_blk() fails)
and has a name len of 0, then __chk_dentries() will end up stuck:

....
[__chk_dentries: 663] [  4]-[0x0] name[] len[0x0] ino[0x5f1710dc] type[0xd1]
[ASSERT] (sanity_check_nid: 184)  --> nid is not valid. [0x5f1710dc]
[__chk_dentries: 663] [  4]-[0x0] name[] len[0x0] ino[0x5f1710dc] type[0xd1]
[ASSERT] (sanity_check_nid: 184)  --> nid is not valid. [0x5f1710dc]
[__chk_dentries: 663] [  4]-[0x0] name[] len[0x0] ino[0x5f1710dc] type[0xd1]
[ASSERT] (sanity_check_nid: 184)  --> nid is not valid. [0x5f1710dc]
....

This change tries to identify bad dentries:
 - is the ino a valid NID?
 - is the FILE_TYPE a happy type?

Change-Id: I5fb2f3869c96f2c928baaace148de1af102e558e
Signed-off-by: JP Abgrall <jpa@google.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>