iomap: add a flag to bounce buffer direct I/O

iomap: add a flag to bounce buffer direct I/O

Add a new flag that request bounce buffering for direct I/O.  This is
needed to provide the stable pages requirement requested by devices
that need to calculate checksums or parity over the data and allows
file systems to properly work with things like T10 protection
information.  The implementation just calls out to the new bio bounce
buffering helpers to allocate a bounce buffer, which is used for
I/O and to copy to/from it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Tested-by: Anuj Gupta <anuj20.g@samsung.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>