xfs: add xfs_setattr_time

Split out a xfs_setattr_time helper to share code between truncate and
regular setattr similar to xfs_setattr_mode.  I might also have another
caller growing for this in the near future.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Ben Myers <bpm@sgi.com>