LU-12678 socklnd: change ksnd_nthreads to atomic_t

LU-12678 socklnd: change ksnd_nthreads to atomic_t

This variable is treated like an atomic_t, but a global spinlock is
used to protect updates - and also unnecessarily to protect reads.

Change to atomic_t and avoid using the spinlock.

Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: Id94d280875a9e115dc077253c49e97a725dc91e1
Reviewed-on: https://review.whamcloud.com/39121
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Chris Horn <chris.horn@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>