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

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 <[email protected]>
Cc: [email protected]
Signed-off-by: Namjae Jeon <[email protected]>
Signed-off-by: Steve French <[email protected]>