librbd: avoid repeatedly invoking is_zero() in CopyupRequest

librbd: avoid repeatedly invoking is_zero() in CopyupRequest

Invoke is_zero() just once, either in handle_read_from_parent() or
handle_deep_copy().

Initialize m_copyup_is_zero to true to preserve existing behavior:
on an empty bufferlist is_zero() returns true.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit b01133661e3bc1814f66df3de70aefd005965b27)