xfs: online repair support

This is the first part of the nineteenth revision of a patchset that
adds to XFS kernel support for online metadata scrubbing and repair.
There aren't any on-disk format changes.

New for this version is a rebase against 5.3-rc2, integration with the
health reporting subsystem, and the explicit revalidation of all
metadata structures that were rebuilt.

Patch 1 lays the groundwork for scrub types specifying a revalidation
function that will check everything that the repair function might have
rebuilt.  This will be necessary for the free space and inode btree
repair functions, which rebuild both btrees at once.

Patch 2 ensures that the health reporting query code doesn't get in the
way of post-repair revalidation of all rebuilt metadata structures.

Patch 3 creates a new data structure that provides an abstraction of a
big memory array by using linked lists.  This is where we store records
for btree reconstruction.  This first implementation is memory
inefficient and consumes a /lot/ of kernel memory, but lays the
groundwork for the last patch in the set to convert the implementation
to use a (memfd) swap file, which enables us to use pageable memory
without pounding the slab cache.

Patches 4-10 implement reconstruction of the free space btrees, inode
btrees, reference count btrees, inode records, inode forks, inode block
maps, and symbolic links.

Patch 11 implements a new data structure for storing arbitrary key/value
pairs, which we're going to need to reconstruct extended attribute
forks.

Patches 12-14 clean up the block unmapping code so that we will be able
to perform a mass reset of an inode's fork.  This is a key component for
salvaging extended attributes, freeing all the attr fork blocks, and
reconstructing the extended attribute data.

Patch 15 implements extended attribute salvage operations.  There is no
redundant or secondary xattr metadata, so the best we can do is trawl
through the attr leaves looking for intact entities.

Patch 16 augments scrub to rebuild extended attributes when any of the
attr blocks are fragmented.

Patch 17 implements reconstruction of quota blocks.

Patch 18 converts both in-memory array implementations from the clunky
linked list implementation to something resembling C arrays.  The array
data are backed by a (memfd) file, which means that idle data can be
paged out to disk instead of pinning kernel memory.