Fix the netfs_folio tracepoint to handle folios that have a NULL mapping
pointer.  In such a case, just substitute a zero inode number.

Fixes: c38f4e96 ("netfs: Provide func to copy data to pagecache for buffered write")
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/2917423.1727697556@warthog.procyon.org.uk
cc: Jeff Layton <jlayton@kernel.org>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>