drs: Check target object is known after applying objects

drs: Check target object is known after applying objects

Currently we only check that the target object is known at the end of
the transaction (i.e. the .prepare_commit hook). It's too late at this
point to resend the request with GET_TGT. Move this processing earlier
on, after we've applied all the objects (i.e. off the .extended hook).

In reality, we need to perform the checks at both points. I've
split the common code that gets the source/target details out of the
la_entry into a helper function. It's not the greatest function ever,
but seemed to make more sense than duplicating the code.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Garming Sam <garming@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
BUG: https://bugzilla.samba.org/show_bug.cgi?id=12972