btrfs: use non-bh spin_lock in zstd timer callback

This is an optimization for fix fee13fe9 ("btrfs: correct zstd
workspace manager lock to use spin_lock_bh()")

The critical region for wsm.lock is only accessed by the process context and
the softirq context.

Because in the soft interrupt, the critical section will not be
preempted by the soft interrupt again, there is no need to call
spin_lock_bh(&wsm.lock) to turn off the soft interrupt,
spin_lock(&wsm.lock) is enough for this situation.

Signed-off-by: Schspa Shi <schspa@gmail.com>
[ minor comment update ]
Signed-off-by: David Sterba <dsterba@suse.com>