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

If ->NameOffset of smb2_create_req is smaller than Buffer offset of
smb2_create_req, slab-out-of-bounds read can happen from smb2_open.
This patch set the minimum value of the name offset to the buffer offset
to validate name length of smb2_create_req().

Cc: stable@vger.kernel.org
Reported-by: Xuanzhe Yu <yuxuanzhe@outlook.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>