xfs: reformat the xfs_fs_free prototype

The xfs_fs_free prototype formatting is a weird mix of the classic XFS
style and the Linux style.  Fix it up to be consistent.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Message-Id: <20230809220545.1308228-2-hch@lst.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>