replmd: Avoid passing msg to replmd_process_linked_attribute()

replmd: Avoid passing msg to replmd_process_linked_attribute()

We can prevent anyone from inadvertently adding/removing msg->elements[]
in replmd_process_linked_attribute() by just not passing msg into the
function. Currently we only actually need the source DN and a memory
context for reallocating old_el->values.

The warning comment has been moved to a more appropriate place.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>