libdm: use destination size as limit in dm_bit_copy()

libdm: use destination size as limit in dm_bit_copy()

The dm_bit_copy() macro uses the source (bs1) bitset size as the
limit for memcpy:

    memcpy((bs1) + 1, (bs2) + 1, ((*(bs1) / DM_BITS_PER_INT) + 1)..)

This is safe if the destination bitset is smaller than the source,
or if the two bitsets are of the same size.

With a destination that is larger (e.g. when resizing a bitmap to
add more capacity), the memcpy will overrun the source bitset and
set garbage bits in the destination.

There are nine uses of the macro currently (8 in libdm/regex, and
1 in daemons/cmirrord): in each case the two bitsets are always of
equal size so the behaviour is unchanged.

Fix the macro to use bs2's size to simplify resizing bitsets and
avoid the need for another copy macro.