block: fix elevator_get_by_features()

The lookup logic is broken - 'e' will never be NULL, even if the
list is empty. Maintain lookup hit in a separate variable instead.

Fixes: a0958ba7

 ("block: Improve default elevator selection")
Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Jens Axboe <axboe@kernel.dk>