s4:dsdb:acl: Fix LDB flags comparison

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.

Commit 99b805e4cbeec232c65adb1a6f3fb326b55c4496 fixed a similar issue.

Signed-off-by: Jennifer Sutton <jennifersutton@catalyst.net.nz>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>