ext4: fix typo which broke '..' detection in ext4_find_entry()

There should be a check for the NUL character instead of '0'.

Fortunately the only thing that cares about this is NFS serving, which
is why we didn't notice this in the merge window testing.

Reported-by: Phil Carmody <ext-phil.2.carmody@nokia.com>
Signed-off-by: Aaro Koskinen <aaro.koskinen@nokia.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>