[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>

fs/logfs/logfs.h

@@ -727,4 +727,10 @@ static inline struct logfs_area *get_area(struct super_block *sb,
 	return logfs_super(sb)->s_area[(__force u8)gc_level];
 }
 
+static inline void logfs_mempool_destroy(mempool_t *pool)
+{
+	if (pool)
+		mempool_destroy(pool);
+}
+
 #endif

fs/logfs/readwrite.c

@@ -2243,8 +2243,6 @@ void logfs_cleanup_rw(struct super_block *sb)
 	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);
 }

fs/logfs/segment.c

@@ -912,7 +912,7 @@ int logfs_init_areas(struct super_block *sb)
 	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;
 }
 

fs/logfs/super.c

@@ -517,8 +517,8 @@ static void logfs_kill_sb(struct super_block *sb)
 	if (super->s_erase_page)
 		__free_page(super->s_erase_page);
 	super->s_devops->put_device(sb);
-	mempool_destroy(super->s_btree_pool);
-	mempool_destroy(super->s_alias_pool);
+	logfs_mempool_destroy(super->s_btree_pool);
+	logfs_mempool_destroy(super->s_alias_pool);
 	kfree(super);
 	log_super("LogFS: Finished unmounting\n");
 }