KM_SLEEP kmem_cache allocations should not block high priority allocations

Signed-off-by: Richard Yao <ryao@gentoo.org>
spl-kmem: Use smp_mb__after_clear_bit()

We should use smp_mb__after_clear_bit() to ensure that threads being
woken will always see that a bit has been cleared.

Signed-off-by: Richard Yao <ryao@gentoo.org>