blk-mq: streamline blk_mq_make_request

Turn the different ways of merging or issuing I/O into a series of if/else
statements instead of the current maze of gotos.  Note that this means we
pin the CPU a little longer for some cases as the CTX put is moved to
common code at the end of the function.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>