shm: shm_exit scalability fixes

[Jack Miller <millerjo@us.ibm.com>]

All -

This is small set of patches our team has had kicking around for a few versions
internally that fixes tasks getting hung on shm_exit when there are many
threads hammering it at once.

Anton wrote a simple test to cause the issue:

http://ozlabs.org/~anton/junkcode/bust_shm_exit.c

Before applying this patchset, this test code will cause either hanging
tracebacks or pthread out of memory errors.

After this patchset, it will still produce output like:

root@somehost:~# ./bust_shm_exit 1024 160
...
INFO: rcu_sched detected stalls on CPUs/tasks: {} (detected by 116, t=2111
jiffies, g=241, c=240, q=7113)
INFO: Stall ended before state dump start
...

But the task will continue to run along happily, so we consider this an
improvement over hanging, even if it's a bit noisy.

I didn't author these patches, but I'd be happy to take any feedback and
address any issues.

- Jack