fuse: invalidate dir dentry after chmod

fuse: invalidate dir dentry after chmod

commit 5e2b8828ff3d79aca8c3a1730652758753205b61 upstream.

Without "default_permissions" the userspace filesystem's lookup operation
needs to perform the check for search permission on the directory.

If directory does not allow search for everyone (this is quite rare) then
userspace filesystem has to set entry timeout to zero to make sure
permissions are always performed.

Changing the mode bits of the directory should also invalidate the
(previously cached) dentry to make sure the next lookup will have a chance
of updating the timeout, if needed.

Reported-by: Jean-Pierre André <jean-pierre.andre@wanadoo.fr>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
[bwh: Backported to 3.2:
 - Adjust context
 - Open-code d_is_dir()]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>