ksmbd: fix slab-out-of-bounds in smb_strndup_from_utf16()

If ->NameOffset/Length is bigger than ->CreateContextsOffset/Length,
ksmbd_check_message doesn't validate request buffer it correctly.
So slab-out-of-bounds warning from calling smb_strndup_from_utf16()
in smb2_open() could happen. If ->NameLength is non-zero, Set the larger
of the two sums (Name and CreateContext size) as the offset and length of
the data area.

Reported-by: Yang Chaoming <lometsj@live.com>
Cc: stable@vger.kernel.org
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>