block: Simplify bsg complete all

block: Simplify bsg complete all

It took me a few tries to figure out what this code did; lets rewrite
it into a more regular form.

The thing that makes this one 'special' is the BSG_F_BLOCK flag, if
that is not set we're not supposed/allowed to block and should spin
wait for completion.

The (new) io_wait_event() will never see a false condition in case of
the spinning and we will therefore not block.

Cc: Linus Torvalds <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>