percpu-rwsem: use synchronize_sched_expedited

Use synchronize_sched_expedited() instead of synchronize_sched()
to improve mount speed.

This patch improves mount time from 0.500s to 0.013s for Jeff's
test-case.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Reported-and-tested-by: Jeff Chua <jeff.chua.linux@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>