xfs: update for 3.18-rc1

This update contains:
o various cleanups
o log recovery debug hooks
o seek hole/data implementation merge
o extent shift rework to fix collapse range bugs
o various sparse warning fixes
o log recovery transaction processing rework to fix use after free bugs
o metadata buffer IO infrastructuer rework to ensure all buffers under IO have
  valid reference counts
o various fixes for ondisk flags, writeback and zero range corner cases
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