CVE-2022-32746 s4/dsdb/acl: Fix LDB flags comparison

LDB_FLAG_MOD_* values are not actually flags, and the previous
comparison was equivalent to

(el->flags & LDB_FLAG_MOD_MASK) == 0

which is only true if none of the LDB_FLAG_MOD_* values are set, so we
would not successfully return if the element was a DELETE. Correct the
expression to what it was intended to be.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15009

Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
