VOL_LOCK needed when traversing DiskPartitionList

VOL_LOCK needed when traversing DiskPartitionList

VLockPartition sets VOL_LOCK, opens (perhaps creating) the lock file
for a partition, and flock's it.  VUnlockPartition, sets VOL_LOCK, and
closes the lock file fd.

VLockPartition is called from the salvager.  Nothing ever calls
VUnlockPartition.

VPFullUnlock is called every 30 seconds, does NOT set VOL_LOCK, and
happily closes any lock file fd it finds, breaking any lock set with
flock.

Other routines use the VOL_LOCK when traversing DiskPartitionList.

Change-Id: I28aa02488a6b53dc9f9aa9b2053c71d29222bdaf
Reviewed-on: http://gerrit.openafs.org/1213
Reviewed-by: Dan Hyde <drh@umich.edu>
Tested-by: Dan Hyde <drh@umich.edu>
(cherry picked from commit 105481d3348a93d33695fb705f9fa5c8af210e7e)
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-on: http://gerrit.openafs.org/1595
(cherry picked from commit 3c1d60a19ccdc23ecd9eb0ca79bd72816abcd82e)