netfs: Use subreq_counter to allocate subreq debug_index values

Use the subreq_counter in netfs_io_request to allocate subrequest
debug_index values in read ops as well as write ops.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org