vol: Use OSI_NULLSOCKET and not -1 to indicate invalid fssync fd

vol: Use OSI_NULLSOCKET and not -1 to indicate invalid fssync fd

The FSync file descriptor is an osi_socket which has an invalid
value of OSI_NULLSOCKET which is not necessarily -1.  Be sure to
compare against OSI_NULLSOCKET and not -1 when checking an invalid
value.

Change-Id: I5b7531e690ea06046b337222b52a5013c7f8802e
Reviewed-on: http://gerrit.openafs.org/3179
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: Derrick Brashear <shadow@dementia.org>