Fix off by one in zpl_lookup

Fix off by one in zpl_lookup

Doing the following command would return success with zfs creating an orphan
object.

	touch $(for i in $(seq 256); do printf "n"; done)

The funny thing is that this will only work once for each directory, because
after upgraded to fzap, zfs_lookup would fail properly since it has additional
length check.

Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #5768