Btrfs: drop unused variable in block_alloc_rsv

{

	struct btrfs_block_rsv *block_rsv;

	struct btrfs_fs_info *fs_info = root->fs_info;

	u64 alloc_target;

	block_rsv = kmalloc(sizeof(*block_rsv), GFP_NOFS);

	if (!block_rsv)

		return NULL;

	btrfs_init_block_rsv(block_rsv);

	alloc_target = btrfs_get_alloc_profile(root, 0);

	block_rsv->space_info = __find_space_info(fs_info,

						  BTRFS_BLOCK_GROUP_METADATA);

	return block_rsv;

}