smb2_server: fix smbd_smb2_request_verify_sizes() for SMB2_OP_WRITE

smb2_server: fix smbd_smb2_request_verify_sizes() for SMB2_OP_WRITE

Writes with a length of 0 are allowed.

The readfile related check we had before was not really useful
as min_dyn_len can only every be 0 or 1 (and for SMB2_OP_WRITE it's
always 1). So we checked
  if (unread_bytes > 0) {
     if (unread_bytes < 1) {
       return error;
     }
  }

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14361

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>