ext4: remove an unneeded NULL check in __ext4_check_dir_entry()

We dereference "bh" unconditionally a couple lines down to find
"by->b_size".  This function is never called with a NULL "bh" so I have
removed the check.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>