xfs_io: rework includes for statx structures

xfs_io: rework includes for statx structures

Only include the kernel's linux/stat.h headers if we haven't
already picked up statx bits from glibc, to avoid redefinition.

Suggested-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Tested-by: Bill O'Donnell <billodo@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Bill O'Donnell <billodo@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>