dm-integrity: don't modify bio's immutable bio_vec in integrity_metadata()

__bio_for_each_segment assumes that the first struct bio_vec argument
doesn't change - it calls "bio_advance_iter_single((bio), &(iter),
(bvl).bv_len)" to advance the iterator. Unfortunately, the dm-integrity
code changes the bio_vec with "bv.bv_len -= pos". When this code path
is taken, the iterator would be out of sync and dm-integrity would
report errors. This happens if the machine is out of memory and
"kmalloc" fails.

Fix this bug by making a copy of "bv" and changing the copy instead.

Fixes: 7eada909 ("dm: add integrity target")
Cc: stable@vger.kernel.org	# v4.12+
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>