netfs: Rename the netfs_io_request cleanup op and give it an op pointer

The netfs_io_request cleanup op is now always in a position to be given a
pointer to a netfs_io_request struct, so this can be passed in instead of
the mapping and private data arguments (both of which are included in the
struct).

So rename the ->cleanup op to ->free_request (to match ->init_request) and
pass in the I/O pointer.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com