afs: do not allow two shutdown sequences in parallel

afs: do not allow two shutdown sequences in parallel

Often, ‘afsd -shutdown’ is called right after ‘umount’.
Both commands hold the glock before calling ‘afs_shutdown’.
However, one of the functions called by 'afs_shutdown', namely,
‘afs_FlushVCBs’, might drop the glock when the global
'afs_shuttingdown' is still equal to 0. As a result, a scenario
with two shutdown sequences proceeding in parallel is possible.

To fix the problem, the global ‘afs_shuttingdown’ is used as an
enumerated type to make sure that the second thread will not run
‘afs_shutdown’ while the first one is stuck inside ‘afs_FlushVCBs’.

Change-Id: Iffa89d82278b0df5fb90fc35608af66d8e8db29e
Reviewed-on: http://gerrit.openafs.org/12016
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams <3chas3@gmail.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>