blkdev.h gets included in various places outside the block layer just
for struct blk_plug and related plugging functions.

Split blk_plug into a separate helper to reduce the amount of code
that needs to get rebuilt when blkdev.h changes and to slightly
reduce compile times.

In io_uring this requires pulling in a few other headers explicitly that
previously were implicitly included through blkdev.h.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Christian Brauner (Amutable) <brauner@kernel.org>