block: make get_request[_wait]() fail if queue is dead

Currently get_request[_wait]() allocates request whether queue is dead
or not.  This patch makes get_request[_wait]() return NULL if @q is
dead.  blk_queue_bio() is updated to fail the submitted bio if request
allocation fails.  While at it, add docbook comments for
get_request[_wait]().

Note that the current code has rather unclear (there are spurious DEAD
tests scattered around) assumption that the owner of a queue
guarantees that no request travels block layer if the queue is dead
and this patch in itself doesn't change much; however, this will allow
fixing the broken assumption in the next patch.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
block: reorganize throtl_get_tg() and blk_throtl_bio()

blk_throtl_bio() and throtl_get_tg() have rather unusual interface.

* throtl_get_tg() returns pointer to a valid tg or ERR_PTR(-ENODEV),
  and drops queue_lock in the latter case.  Different locking context
  depending on return value is error-prone and DEAD state is scheduled
  to be protected by queue_lock anyway.  Move DEAD check inside
  queue_lock and return valid tg or NULL.

* blk_throtl_bio() indicates return status both with its return value
  and in/out param **@bio.  The former is used to indicate whether
  queue is found to be dead during throtl processing.  The latter
  whether the bio is throttled.

  There's no point in returning DEAD check result from
  blk_throtl_bio().  The queue can die after blk_throtl_bio() is
  finished but before make_request_fn() grabs queue lock.

  Make it take *@bio instead and return boolean result indicating
  whether the request is throttled or not.

This patch doesn't cause any visible functional difference.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Conflicts:
	block/blk-core.c
	block/blk-throttle.c
	block/blk.h