block: flip iter directions in blk_rq_integrity_map_user()

blk_rq_integrity_map_user() creates the ubuf iter with ITER_DEST for
write-direction operations and ITER_SOURCE for read-direction ones.
This is backwards; writes use the user buffer as a source for metadata
and reads use it as a destination. Switch to the rq_data_dir() helper,
which maps writes to ITER_SOURCE (WRITE) and reads to ITER_DEST(READ).

Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
Fixes: fe8f4ca7 ("block: modify bio_integrity_map_user to accept iov_iter as argument")
Link: https://lore.kernel.org/r/20250603184752.1185676-1-csander@purestorage.com


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