ldb_tdb: Rework ltdb_modify_internal() to use ltdb_search_dn1() internally

ldb_tdb: Rework ltdb_modify_internal() to use ltdb_search_dn1() internally

This avoids duplicate code and allows us to use the allocation-avoiding
LDB_UNPACK_DATA_FLAG_NO_DATA_ALLOC flag.

We can not use LDB_UNPACK_DATA_FLAG_NO_VALUES_ALLOC as el2->values
is talloc_realloc()ed in the routine.

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Tue Aug 29 11:13:50 CEST 2017 on sn-devel-144