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’.

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>
(cherry picked from commit 70fd9bc6dcc79cb25e98cdcfd0f085c4bf4f310a)

Change-Id: I073d1914a7daa858a78305ff154074f2a51a9f5f
Reviewed-on: https://gerrit.openafs.org/12179
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Marcio Brito Barbosa <mbarbosa@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>