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 check_add_overflow() to avoid malformed ACL

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

pndacl->size = cpu_to_le16(le16_to_cpu(pndacl->size) + size);

This produces an on-wire NT ACL whose pndacl->size under-reports the
bytes actually written by the preceding fill_ace_for_sid()/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 -= ace_sz` / `size -= nt_ace_size`) 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 check_add_overflow() 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 (transport_ipc.c) or break without
consuming the overflowed destination value afterward (buildid.c).