Optimize spl_rwsem_is_locked()

The spl_rwsem_is_locked() compatibility function has been observed
to be a hot spot. The root cause of this is that we must check the
rwsem activity under the rwsem->wait_lock to avoid a race. When
the lock is busy significant contention can occur.

The upstream kernel fix for this race had the insight that by using
spin_trylock_irqsave() this contention could be avoided. When the
lock is contended it's reasonable to return that it is locked.

This change updates the SPLs implemention to be like the upstream
kernel. Since the kernel code has been in use for years now this
a low risk change.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>