lockd: remove redundant check on block

A null check followed by a return is being performed already, so block
is always non-null at the second check on block, hence we can remove
this redundant null-check (Detected by PVS-Studio).  Also re-work
comment to clean up a check-patch warning.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>