generic/247: filter out expected XFS warnings for mixed mmap/direct I/O

generic/247: filter out expected XFS warnings for mixed mmap/direct I/O

generic/247 reproduces some of the same, expected warnings from XFS
as generic/095. These warnings occur due to mixed buffered/mapped
I/O racing with direct I/O to the same file.

generic/095 contains a custom dmesg filter to prevent test failure
in the event of such warnings. Lift the helper from generic/095 to
common/xfs and reuse it in generic/247 to implement the same
behavior.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>