fail_make_request: cleanup should_fail_request

This changes should_fail_request() to more usable wrapper function of
should_fail().  It can avoid putting #ifdef CONFIG_FAIL_MAKE_REQUEST in
the middle of a function.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>