SUNRPC: Add a helper to allow pNFS drivers to selectively cancel RPC calls

Add the helper rpc_cancel_tasks(), which uses a caller-defined selection
function to define a set of in-flight RPC calls to cancel. This is
mainly intended for pNFS drivers which are subject to a layout recall,
and which may therefore want to cancel all pending I/O using that layout
in order to redrive it after the layout recall has been satisfied.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>