fuse: fix fileattr op failure

The fileattr API conversion broke lsattr on ntfs3g.

Previously the ioctl(... FS_IOC_GETFLAGS) returned an EINVAL error, but
after the conversion the error returned by the fuse filesystem was not
propagated back to the ioctl() system call, resulting in success being
returned with bogus values.

Fix by checking for outarg.result in fuse_priv_ioctl(), just as generic
ioctl code does.

Reported-by: Jean-Pierre André <jean-pierre.andre@wanadoo.fr>
Fixes: 72227eac

 ("fuse: convert to fileattr")
Cc: <stable@vger.kernel.org> # v5.13
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>