xfs: only bother with sync_filesystem during readonly remount

In commit 02b9984d, we pushed a sync_filesystem() call from the VFS
into xfs_fs_remount.  The only time that we ever need to push dirty file
data or metadata to disk for a remount is if we're remounting the
filesystem read only, so this really could be moved to xfs_remount_ro.

Once we've moved the call site, actually check the return value from
sync_filesystem.

Fixes: 02b9984d ("fs: push sync_filesystem() down to the file system's remount_fs()")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>