ksmbd: validate request buffer size in smb2_allocate_rsp_buf()

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

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.