mirror: Update signature of mirror_clip_sectors()

mirror: Update signature of mirror_clip_sectors()

Rather than having a void function that modifies its input
in-place as the output, change the signature to reduce a layer
of indirection and return the result.

Suggested-by: John Snow <jsnow@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>