swim3: convert to blk-mq

Pretty simple conversion. grab_drive() could probably be replaced by
some freeze/quiesce incantation, but I left it alone, and just used
freeze/quiesce for eject. Compile-tested only.

Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Omar Sandoval <osandov@fb.com>

Converted to blk_mq_init_sq_queue().

Signed-off-by: Jens Axboe <axboe@kernel.dk>