dlm: fix missing check in validate_lock_args

This patch adds a additional check if lkb->lkb_wait_count is non zero as
it is done in validate_unlock_args() to check if any operation is in
progress. While on it add a comment taken from validate_unlock_args() to
signal what the check is doing.

There might be no changes because if lkb->lkb_wait_type is non zero
implies that lkb->lkb_wait_count is non zero. However we should add the
check as it does validate_unlock_args().

Signed-off-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: David Teigland <teigland@redhat.com>