Btrfs: return free space to global_rsv as much as possible

@full is not protected within global_rsv.lock, so we may think global_rsv
is already full but in fact it's not, so we miss the opportunity to return
free space to global_rsv directly when we release other block_rsvs.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/extent-tree.c

@@ -4674,7 +4674,7 @@ void btrfs_block_rsv_release(struct btrfs_root *root,
 			     u64 num_bytes)
 {
 	struct btrfs_block_rsv *global_rsv = &root->fs_info->global_block_rsv;
-	if (global_rsv->full || global_rsv == block_rsv ||
+	if (global_rsv == block_rsv ||
 	    block_rsv->space_info != global_rsv->space_info)
 		global_rsv = NULL;
 	block_rsv_release_bytes(root->fs_info, block_rsv, global_rsv,