Allow copy offload (CopyChunk) across shares

FSCTL_SRV_COPYCHUNK_WRITE only requires that the source and target
be on the same server (not the same volume or same share),
so relax the existing check (which required them to be on
the same share). Note that this works to Windows (and presumably
most other NAS) but Samba requires that the source
and target be on the same share.  Moving a file across
shares is a common use case and can be very heplful (100x faster).

Signed-off-by: Steve French <steve.french@primarydata.com>
Reviewed-by: David Disseldorp <ddiss@samba.org>