netfs: Provide readahead and readpage netfs helpers

Add a pair of helper functions:

 (*) netfs_readahead()
 (*) netfs_readpage()

to do the work of handling a readahead or a readpage, where the page(s)
that form part of the request may be split between the local cache, the
server or just require clearing, and may be single pages and transparent
huge pages.  This is all handled within the helper.

Note that while both will read from the cache if there is data present,
only netfs_readahead() will expand the request beyond what it was asked to
do, and only netfs_readahead() will write back to the cache.

netfs_readpage(), on the other hand, is synchronous and only fetches the
page (which might be a THP) it is asked for.

The netfs gives the helper parameters from the VM, the cache cookie it
wants to use (or NULL) and a table of operations (only one of which is
mandatory):

 (*) expand_readahead() [optional]

     Called to allow the netfs to request an expansion of a readahead
     request to meet its own a...