xfs: flush for older, xfs specific ioctls

837a6e7f5cdb ("fs: add generic UNRESVSP and ZERO_RANGE ioctl handlers") changed
ioctls XFS_IOC_UNRESVSP XFS_IOC_UNRESVSP64 and XFS_IOC_ZERO_RANGE to be generic
instead of xfs specific.

Because of this change, 36f11775 ("xfs: properly serialise fallocate against
AIO+DIO") needed adaptation, as 5.4 still uses the xfs specific ioctls.

Without this, xfstests xfs/242 and xfs/290 fail. Both of these tests test
XFS_IOC_ZERO_RANGE.

Fixes: 36f11775 ("xfs: properly serialise fallocate against AIO+DIO")
Tested-by: Andy Strohman <astroh@amazon.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>