cifsd: Fix an error code in smb2_read()

This code is assigning the wrong variable to "err" so it returns
zero/success instead of -ENOMEM.

Fixes: 788b6f45c1d2 ("cifsd: add server-side procedures for SMB3")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Steve French <stfrench@microsoft.com>