ksmbd: reserve space for the UTF-16 terminator in stream info

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

ksmbd: reserve space for the UTF-16 terminator in stream info

smb_strtoUTF16() always writes a terminating zero UTF-16 unit after the
converted characters, but get_file_stream_info() budgets only
sizeof(struct smb2_file_stream_info) + streamlen * 2 for a stream entry
and sizeof(struct smb2_file_stream_info) + 7 * 2 for the trailing
"::$DATA" entry. When an entry exactly fills the 448-byte small response
buffer, the terminator is written two bytes past the allocation.

Reserve one extra UTF-16 unit in both budget checks. The terminator is
not part of the wire format, so the reported lengths are unchanged.