xfs: stage repair information in pageable memory [v26.1]

In general, online repair of an indexed record set walks the filesystem
looking for records.  These records are sorted and bulk-loaded into a
new btree.  To make this happen without pinning gigabytes of metadata in
memory, first create an abstraction ('xfile') of memfd files so that
kernel code can access paged memory, and then an array abstraction
('xfarray') based on xfiles so that online repair can create an array of
new records without pinning memory.

These two data storage abstractions are critical for repair of space
metadata -- the memory used is pageable, which helps us avoid pinning
kernel memory and driving OOM problems; and they are byte-accessible
enough that we can use them like (very slow and programmatic) memory
buffers.

Later patchsets will build on this functionality to provide blob storage
and btrees.

This has been running on the djcloud for years with no problems.  Enjoy!

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
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