libafs: afs_SetupVolSlot function

Move the code block to get and setup volume slots out of
afs_SetupVolume to a new local function called afs_SetupVolSlot.
This new function acquires the afs_xvolume lock and releases it
before returning.

Change-Id: I1bd33f13e0525f9ff050d7e161cf29a511e5c4b8
Reviewed-on: http://gerrit.openafs.org/10879
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: D Brashear <shadow@your-file-system.com>