kclient: evaluate/fix/add lazio support in the kernel
ceph-fuse now supports lazyio but I don't believe we ever checked what needed to be done for the kernel client.
Also, I got a question from Dan @ CERN about how lazyio interacts with the page cache (or for ceph-fuse, the osdc cache). In particular, if a client 1 reads some file blocks and another client 2 updates those blocks, how does client 1 expire its page cache? Does it need to reopen the file? It seems we're missing the lazyio_propagate/lazyio_synchronize interfaces to resolve this which do exist in the Client userspace library , but untested (#40283).
Targeting v15.0.0 just so this ticket is visible in searches.