afs: Don't invalidate callback if AFS_VNODE_DIR_VALID not set

Don't invalidate the callback promise on a directory if the
AFS_VNODE_DIR_VALID flag is not set (which indicates that the directory
contents are invalid, due to edit failure, callback break, page reclaim).

The directory will be reloaded next time the directory is accessed, so
clearing the callback flag at this point may race with a reload of the
directory and cancel it's recorded callback promise.

Fixes: f3ddee8d ("afs: Fix directory handling")
Signed-off-by: David Howells <dhowells@redhat.com>