xfs: online repair of inode data

For the third part of the twenty-first revision of the online repair
patchset, we implement repair of extended attribute data.

Patch 1 implements a new data structure for storing arbitrary key/value
pairs, which we're going to need to reconstruct extended attribute
forks.

Patches 2-4 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 5 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 6 augments scrub to rebuild extended attributes when any of the
attr blocks are fragmented.