wait: add wait_event_cmd()

Add a new API wait_event_cmd(). It's a variant of wait_even() with two
commands executed. One is executed before sleep, another after sleep.

Modified to match use wait.h approach based on suggestion by
Peter Zijlstra <peterz@infradead.org> - neilb

Signed-off-by: Shaohua Li <shli@fusionio.com>
Signed-off-by: NeilBrown <neilb@suse.de>