CVE-2026-68099: ksmbd: restore DACL size on check_add_overflow() to avoid malformed ACL

In the Linux kernel, the following vulnerability has been resolved:

ksmbd: restore DACL size on checkaddoverflow() to avoid malformed ACL

checkaddoverflow() unconditionally writes the truncated sum into d even on overflow, per its contract in include/linux/overflow.h. The four checkaddoverflow() guards in setposixaclentriesdacl() and setntacldacl() break out of the ACE-building loops on overflow, but the truncated size is then consumed downstream at the end of setntacldacl():

pndacl->size = cputole16(le16tocpu(pndacl->size) + size);

This produces an on-wire NT ACL whose pndacl->size under-reports the bytes actually written by the preceding fillaceforsid()/memcpy() calls, yielding a malformed ACL that can trigger out-of-bounds reads when re-parsed by clients or ksmbd itself.

Restore size to its pre-addition value on each overflow branch (via size -= acesz / size -= ntacesize) so that after the break, size once again holds the cumulative size of the successfully-written ACEs. The committed ACL is then truncated-but-self-consistent rather than malformed.

The ksmbd DACL builders are the only checkaddoverflow() sites found where an overflow path breaks out of a loop and the destination value is consumed afterward. The other nearby break-style cases either return -EINVAL on overflow (transportipc.c) or break without consuming the overflowed destination value afterward (buildid.c).