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_map.c b/fs/btrfs/extent_map.c
index ac606f0..8d1364d 100644
--- a/fs/btrfs/extent_map.c
+++ b/fs/btrfs/extent_map.c
@@ -11,7 +11,7 @@
 
 int __init extent_map_init(void)
 {
-	extent_map_cache = kmem_cache_create("extent_map",
+	extent_map_cache = kmem_cache_create("btrfs_extent_map",
 			sizeof(struct extent_map), 0,
 			SLAB_RECLAIM_ACCOUNT | SLAB_MEM_SPREAD, NULL);
 	if (!extent_map_cache)