xfs: convert buffer cache to use high order folios

Now that we have the buffer cache using the folio API, we can extend
the use of folios to allocate high order folios for multi-page
buffers rather than an array of single pages that are then vmapped
into a contiguous range.

This creates a new type of single folio buffers that can have arbitrary
order in addition to the existing multi-folio buffers made up of many
single page folios that get vmapped.  The single folio is for now
stashed into the existing b_pages array, but that will go away entirely
later in the series and remove the temporary page vs folio typing issues
that only work because the two structures currently can be used largely
interchangeable.

The code that allocates buffers will optimistically attempt a high
order folio allocation as a fast path if the buffer size is a power
of two and thus fits into a folio. If this high order allocation
fails, then we fall back to the existing multi-folio allocation
code. This now forms the slow allocation path, and hopefully will be
largely unused in normal conditions except for buffers with size
that are not a power of two like larger remote xattrs.

This should improve performance of large buffer operations (e.g.
large directory block sizes) as we should now mostly avoid the
expense of vmapping large buffers (and the vmap lock contention that
can occur) as well as avoid the runtime pressure that frequently
accessing kernel vmapped pages put on the TLBs.

Based on a patch from Dave Chinner <dchinner@redhat.com>, but mutilated
beyond recognition.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>

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