vfs: Remove the const from dir_context::actor

Remove the const marking from the actor function pointer in the dir_context
struct.  The const prevents the structure from being used as part of a
kmalloc'd object as it makes the compiler require that the actor member be
set at object initialisation time (or not at all), incuring something like
the following error if you try and set it later:

	fs/afs/dir.c:556:20: error: assignment of read-only member 'actor'

Marking the member const like this adds very little in the way of sanity
checking as the type checking system is likely to provide sufficient - and
if not, the kernel is very likely to oops repeatably in this case.

Fixes: ac6614b7 ("[readdir] constify ->actor")
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Al Viro <viro@zeniv.linux.org.uk>