ksmbd: validate request buffer size in smb2_allocate_rsp_buf()

ksmbd: validate request buffer size in smb2_allocate_rsp_buf()

The response buffer should be allocated in smb2_allocate_rsp_buf
before validating request. But the fields in payload as well as smb2 header
is used in smb2_allocate_rsp_buf(). This patch add simple buffer size
validation to avoid potencial out-of-bounds in request buffer.

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