DAFS: Wait for exclusive ops in VFreeBitMapEntry_r

DAFS: Wait for exclusive ops in VFreeBitMapEntry_r

VAllocBitmapEntry_r puts the volume in an exclusive state and drops
VOL_LOCK when traversing the volume bitmap and updating the bitmap.
So, VFreeBitMapEntry_r must ensure the volume is not in an exclusive
state, to make sure that VAllocBitmapEntry_r is not updating the
bitmap at the same time. Do so, by waiting for the volume to come out
of exclusive state at the beginning of VFreeBitMapEntry_r.

Change-Id: I5fdd344e4d9d12451fd65a767fa5672c8be70a39
Reviewed-on: http://gerrit.openafs.org/4058
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>