staging: lustre: fid: use wait_event_cmd()

Rather than open-coding a wait event loop twice,
use wait_event_cmd() to wait, dropping the spinlock
over schedule().
This does require duplicating part of the wait
condition, but that is just three tests on values that
are in registers or in cache, so the cost is small
and the increased readability is large.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>