[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/readwrite.c b/fs/logfs/readwrite.c
index 7e0c39c..aca6c56 100644
--- a/fs/logfs/readwrite.c
+++ b/fs/logfs/readwrite.c
@@ -2243,8 +2243,6 @@
 	struct logfs_super *super = logfs_super(sb);
 
 	destroy_meta_inode(super->s_segfile_inode);
-	if (super->s_block_pool)
-		mempool_destroy(super->s_block_pool);
-	if (super->s_shadow_pool)
-		mempool_destroy(super->s_shadow_pool);
+	logfs_mempool_destroy(super->s_block_pool);
+	logfs_mempool_destroy(super->s_shadow_pool);
 }