xfs: update for 3.19-rc1

This update contains:
o more on-disk format header consolidation
o move some structures shared with userspace to libxfs
o new per-mount workqueue to fix for deadlocks between nested loop
  mounted filesystems
o various bug fixes for ENOSPC, stats, quota off and preallocation
o a bunch of compiler warning fixes for set-but-unused variables
o various code cleanups
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