dm integrity: only allocate recalculate buffer when needed

dm-integrity preallocated 8MiB buffer for recalculating in the
constructor and freed it in the destructor. This wastes memory when
the user has many dm-integrity devices.

Fix dm-integrity so that the buffer is only allocated when
recalculation is in progress; allocate the buffer at the beginning of
integrity_recalc() and free it at the end.

Note that integrity_recalc() doesn't hold any locks when allocating
the buffer, so it shouldn't cause low-memory deadlock.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>