btrfs: polish names of kmem caches

Usecase:

  watch 'grep btrfs < /proc/slabinfo'

easy to watch all caches in one go.

Signed-off-by: David Sterba <dsterba@suse.cz>
diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 4a41b17..3ad84f5 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -64,13 +64,13 @@
 
 int __init extent_io_init(void)
 {
-	extent_state_cache = kmem_cache_create("extent_state",
+	extent_state_cache = kmem_cache_create("btrfs_extent_state",
 			sizeof(struct extent_state), 0,
 			SLAB_RECLAIM_ACCOUNT | SLAB_MEM_SPREAD, NULL);
 	if (!extent_state_cache)
 		return -ENOMEM;
 
-	extent_buffer_cache = kmem_cache_create("extent_buffers",
+	extent_buffer_cache = kmem_cache_create("btrfs_extent_buffer",
 			sizeof(struct extent_buffer), 0,
 			SLAB_RECLAIM_ACCOUNT | SLAB_MEM_SPREAD, NULL);
 	if (!extent_buffer_cache)