[LogFS] Prevent mempool_destroy NULL pointer dereference

It would probably be better to just accept NULL pointers in
mempool_destroy().  But for the current -rc series let's keep things
simple.

This patch was lost in the cracks for a while.
Kevin Cernekee <cernekee@gmail.com> had to rediscover the problem and
send a similar patch because of it. :(

Signed-off-by: Joern Engel <joern@logfs.org>
diff --git a/fs/logfs/segment.c b/fs/logfs/segment.c
index 02db22e..8c82fe0 100644
--- a/fs/logfs/segment.c
+++ b/fs/logfs/segment.c
@@ -912,7 +912,7 @@
 	for (i--; i >= 0; i--)
 		free_area(super->s_area[i]);
 	free_area(super->s_journal_area);
-	mempool_destroy(super->s_alias_pool);
+	logfs_mempool_destroy(super->s_alias_pool);
 	return -ENOMEM;
 }