netfs: Update i_blocks when write committed to pagecache

Update i_blocks when i_size is updated when we finish making a write to the
pagecache to reflect the amount of space we think will be consumed.

This maintains cifs commit dbfdff40

 ("smb3:
update allocation size more accurately on write completion") which would
otherwise be removed by the cifs part of the netfs writeback rewrite.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: Steve French <sfrench@samba.org>
cc: Shyam Prasad N <nspmangalore@gmail.com>
cc: Rohith Surabattula <rohiths.msft@gmail.com>
cc: linux-cifs@vger.kernel.org
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
cc: linux-mm@kvack.org