ext4: Fix handling of direntlen in unlink_filename

ext4: Fix handling of direntlen in unlink_filename

The direntlen checks were quite bogus, i.e. the loop termination used
"len + offset == blocksize" (exact match only), and checked for a
direntlen less than 0. The latter can never happen as the len is
unsigned, this has been reported by Coverity, CID 153384.

Use the same code as in search_dir for directory traversal. This code
has the correct checks for direntlen >= sizeof(struct dirent), and
offset < blocksize.

Signed-off-by: Stefan Brüns <[email protected]>
Reported-by: Coverity (CID: 153383, 153384)
Reviewed-by: Tom Rini <[email protected]>
Reviewed-by: Lukasz Majewski <[email protected]>