replmd: Split up replmd_verify_linked_attribute() into src/target checks

replmd: Split up replmd_verify_linked_attribute() into src/target checks

Refactor replmd_verify_linked_attribute() so we split out the link
attribute source/target checks. This patch should not alter
functionality.

The source object check has been moved out to where
replmd_verify_linked_attribute() was called.

replmd_verify_linked_attribute() has been renamed, as it's now only
checking the link target.

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