dm crypt: don't overallocate the integrity tag space

bio_sectors() returns the value in the units of 512-byte sectors (no
matter what the real sector size of the device).  dm-crypt multiplies
bio_sectors() by on_disk_tag_size to calculate the space allocated for
integrity tags.  If dm-crypt is running with sector size larger than
512b, it allocates more data than is needed.

Device Mapper trims the extra space when passing the bio to
dm-integrity, so this bug didn't result in any visible misbehavior.
But it must be fixed to avoid wasteful memory allocation for the block
integrity payload.

Fixes: ef43aa38 ("dm crypt: add cryptographic data integrity protection (authenticated encryption)")
Cc: stable@vger.kernel.org # 4.12+
Reported-by: Milan Broz <mbroz@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>