dm persistent data: fix memory allocation failure

kmalloc is unreliable when allocating more than 8 pages of memory. It may
fail when there is plenty of free memory but the memory is fragmented.
Zdenek Kabelac observed such failure in his tests.

This commit changes kmalloc to kvmalloc - kvmalloc will fall back to
vmalloc if the large allocation fails.

Signed-off-by: Mikulas Patocka <[e-mail]>
Reported-by: Zdenek Kabelac <[e-mail]>
Reviewed-by: Mike Snitzer <[e-mail]>
Cc: [e-mail]

drivers/md/persistent-data/dm-space-map-metadata.c

···
 {
 	struct sm_metadata *smm = container_of(sm, struct sm_metadata, sm);
 
-	kfree(smm);
+	kvfree(smm);
 }
 
 static int sm_metadata_get_nr_blocks(struct dm_space_map *sm, dm_block_t *count)
···
 {
 	struct sm_metadata *smm;
 
-	smm = kmalloc(sizeof(*smm), GFP_KERNEL);
+	smm = kvmalloc(sizeof(*smm), GFP_KERNEL);
 	if (!smm)
 		return ERR_PTR(-ENOMEM);