xfs: online repair of inode data

For the third part of the twentieth revision of the online repair
patchset, we implement repair of symbolic links and extended attribute
data.

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.