netfs: Fix readahead synchronisation issues by loading all folios upfront

In the Linux kernel, the following vulnerability has been resolved:

netfs: Fix readahead synchronisation issues by loading all folios upfront

There are some synchronisation issues that derive from the app thread
adding more folios to the rolling buffer whilst the collector thread is
looking at them or trying to clear them, such as determining the setting of
front_folio_order when the next folio hasn't been added yet,

The reason for the rolling buffer approach is that loading the buffer
upfront and then dropping all the refs just acquired is quite a slow
operation, and loading progressively allows some of the cost to be deferred
until after at least some of the I/O is started.

Instead, a better way is to load all the folios into the rolling buffer
upfront - and then drop the refs later, once the I/O is in progress. (Even
better would be for the refs not to be there at all.)

Fix this by changing the rolling buffer loader to load all the folios
selected by the VM for readahead upfront into the folio queue. The folio
queue is allocated a batch worth at a time as we don't know how many folios
are involved (the readahead_control struct, alas, has a page count, not a
folio count).

The folio refs acquired from readahead are then dropped in bulk once the
first subrequest is dispatched as it's quite a slow operation. The
collector waits for NETFS_RREQ_NEED_PUT_RA_REFS to be cleared so that it
doesn't unlock folios before the xarray has been scanned for them.

This simplifies the buffer handling later and isn't noticeably slower as
the xarray doesn't need to be modified and the folios are all already
pre-locked.