afs: Make read subreqs async

Perform AFS read subrequests in a work item rather than in the calling
thread.  For normal buffered reads, this will allow the calling thread to
copy data from the pagecache to the application at the same time as the
demarshalling thread is shovelling data from skbuffs into the pagecache.

This will also allow the RA mark to trigger a new read before we've
finished shovelling the data from the current one.

Note: This would be a bit safer if the FS.FetchData RPC ops returned the
metadata (including the data version number) before returning the data.
This would allow me to flush the pagecache before installing the new data.

In future, it may be possible to asynchronously flush the pagecache either
side of the region being read.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: linux-afs@lists.infradead.org
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Link: https://lore.kernel.org/r/20240814203850.2240469-19-dhowells@redhat.com/

 # v2
Signed-off-by: Christian Brauner <brauner@kernel.org>