xfs: remove a superfluous s_fs_info NULL check in xfs_fs_put_super

->put_super is only called when sb->s_root is set, and thus when
fill_super succeeds.  Thus drop the NULL check that can't happen in
xfs_fs_put_super.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Message-Id: <20230809220545.1308228-3-hch@lst.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>