netfs: Limit the minimum trigger for progress reporting

For really big read RPC ops that span multiple folios, netfslib allows the
filesystem to give progress notifications to wake up the collector thread
to do a collection of folios that have now been fetched, even if the RPC is
still ongoing, thereby allowing the application to make progress.

The trigger for this is that at least one folio has been downloaded since
the clean point.  If, however, the folios are small, this means the
collector thread is constantly being woken up - which has a negative
performance impact on the system.

Set a minimum trigger of 256KiB or the size of the folio at the front of
the queue, whichever is larger.

Also, fix the base to be the stream collection point, not the point at
which the collector has cleaned up to (which is currently 0 until something
has been collected).

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Paulo Alcantara <pc@manguebit.org>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org