dm: store dm_target_io in bio front_pad

dm: store dm_target_io in bio front_pad

Use the recently-added bio front_pad field to allocate struct dm_target_io.

Prior to this patch, dm_target_io was allocated from a mempool. For each
dm_target_io, there is exactly one bio allocated from a bioset.

This patch merges these two allocations into one allocation: we create a
bioset with front_pad equal to the size of dm_target_io so that every
bio allocated from the bioset has sizeof(struct dm_target_io) bytes
before it. We allocate a bio and use the bytes before the bio as
dm_target_io.

_tio_cache is removed and the tio_pool mempool is now only used for
request-based devices.

This idea was introduced by Kent Overstreet.

Signed-off-by: Mikulas Patocka <[email protected]>
Cc: Kent Overstreet <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: [email protected]
Cc: Vivek Goyal <[email protected]>
Cc: Bill Pemberton <[email protected]>
Signed-off-by: Alasdair G Kergon <[email protected]>