cifs: Fix wrong return value checking when GETFLAGS

The return value of CIFSGetExtAttr is negative, should be checked
with -EOPNOTSUPP rather than EOPNOTSUPP.

Fixes: 64a5cfa6 ("Allow setting per-file compression via SMB2/3")
Signed-off-by: Zhang Xiaoxu <zhangxiaoxu5@huawei.com>
Signed-off-by: Steve French <stfrench@microsoft.com>