cifsd: fix a IS_ERR() vs NULL bug

The smb_direct_alloc_sendmsg() function never returns NULL, it only
returns error pointers so the check needs to be updated.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Steve French <stfrench@microsoft.com>