bdev: use spinlock instead of mutex

SPDK threads generally run on dedicated cores and locks should be rarely
contended. Thus, putting a thread to sleep while waiting on a mutex does
not free up CPU cycles for other pthreads or processes. Even when
running in interrupt mode, lock contention should be low enough that
spinlocks are a net win by avoiding context switches.

Signed-off-by: Mike Gerdts <mgerdts@nvidia.com>
Change-Id: I6e2e78b2835bbadb56bbec34918d998d75280dfd
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/15438


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Shuhei Matsumoto <smatsumoto@nvidia.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Aleksey Marchuk <alexeymar@nvidia.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>