dm vdo: update vdo_allocate_extended to take a field name, no types

dm vdo: update vdo_allocate_extended to take a field name, no types

All of VDO's "extended" allocations use a flexible array field at the
end of the allocated structure. We can infer the struct type from the
supplied pointer. Replacing the array field type with the field name
lets us use struct_size from overflow.h to compute the size instead of
the local __vdo_do_allocation version.

One allocation of bio structures doesn't conform to this pattern,
since the removal of bi_inline_vecs; directly compute the total size
for that case.

Signed-off-by: Ken Raeburn <raeburn@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>