Btrfs: alloc_mutex latency reduction

This releases the alloc_mutex in a few places that hold it for over long
operations.  btrfs_lookup_block_group is changed so that it doesn't need
the mutex at all.
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/ctree.h

@@ -483,6 +483,7 @@ struct btrfs_block_group_cache {
 	struct btrfs_key key;
 	struct btrfs_block_group_item item;
 	struct btrfs_space_info *space_info;
+	spinlock_t lock;
 	u64 pinned;
 	u64 flags;
 	int cached;