DAFS: Wait until preattach to service FSSYNC reqs

DAFS: Wait until preattach to service FSSYNC reqs

Make FSYNC_sync wait until VInit == 2 before serving FSSYNC requests.
This follows the existing similar behavior of waiting until after volume
preattachment is completed before serving RPC requests in DAFS. This
prevents FSSYNC clients from being denied requests simply due to a
volume not having been preattached yet, at the small cost of delaying
how soon we can serve FSSYNC requests.

This does not alter the behavior of non-DAFS, which still only waits
until VInit == 1, since volume attachment takes significantly longer
than preattachment.

Reviewed-on: http://gerrit.openafs.org/563
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>