cifs: use posix_unblock_lock instead of locks_delete_block

commit 66189be74 (CIFS: Fix VFS lock usage for oplocked files) exported
the locks_delete_block symbol. There's already an exported helper
function that provides this capability however, so make cifs use that
instead and turn locks_delete_block back into a static function.

Note that if fl->fl_next == NULL then this lock has already been through
locks_delete_block(), so we should be OK to ignore an ENOENT error here
and simply not retry the lock.

Cc: Pavel Shilovsky <piastryyy@gmail.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Acked-by: J. Bruce Fields <bfields@fieldses.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/cifs/file.c b/fs/cifs/file.c
index 48b29d2..1686e40 100644
--- a/fs/cifs/file.c
+++ b/fs/cifs/file.c
@@ -999,7 +999,7 @@
 		rc = wait_event_interruptible(flock->fl_wait, !flock->fl_next);
 		if (!rc)
 			goto try_again;
-		locks_delete_block(flock);
+		posix_unblock_lock(flock);
 	}
 	return rc;
 }