s4-ldb: use LDB_FLAG_MOD_TYPE() to extract element type from messages

s4-ldb: use LDB_FLAG_MOD_TYPE() to extract element type from messages

The flags field of message elements is part of a set of flags. We had
LDB_FLAG_MOD_MASK for extracting the type, but it was only rarely
being used (only 1 call used it correctly). This adds
LDB_FLAG_MOD_MASK() to make it more obvious what is going on.

This will allow us to use some of the other flags bits for internal
markers on elements

Pair-Programmed-With: Andrew Bartlett <abartlet@samba.org>