blk-mq: add a flags parameter to blk_mq_alloc_request

blk-mq: add a flags parameter to blk_mq_alloc_request

BugLink: http://bugs.launchpad.net/bugs/1531539
We already have the reserved flag, and a nowait flag awkwardly encoded as
a gfp_t.  Add a real flags argument to make the scheme more extensible and
allow for a nicer calling convention.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>
(cherry picked from commit 6f3b0e8bcf3cbb87a7459b3ed018d31d918df3f8)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>