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.

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>
(cherry picked from commit e8991ff8286f151d109bb4f98d885a583e198f83)

Change-Id: I1627ce0b78d10626b7d375546b400459ff1724ca
Reviewed-on: http://gerrit.openafs.org/3885
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>