vol: Add VInit cond var and remove busywaits

vol: Add VInit cond var and remove busywaits

In DAFS, FSYNC_sync was waiting for VInit to reach at least 2 by
looping around pthread_yield(). For a server with a large number of
volumes, it can take a while for volumes to preattach, and so we are
effectively busy-waiting for preattach to finish. This can slow
fileserver startup and peg the cpu.

So instead, add a condition variable for when VInit changes, and wait
on that. Also modify other checkers of VInit to use the cond var.

Change-Id: Icc251fdf7d525d40a9db1938a9cb4d47a74dccba
Reviewed-on: http://gerrit.openafs.org/2648
Tested-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>