xfs: update for 4.2-rc1 This update contains: o A new sparse on-disk inode record format to allow small extents to be used for inode allocation when free space is fragmented. o DAX support. This includes minor changes to the DAX core code to fix problems with lock ordering and bufferhead mapping abuse. o transaction commit interface cleanup o removal of various unnecessary XFS specific type definitions o cleanup and optimisation of freelist preparation before allocation o various minor cleanups o bug fixes for - transaction reservation leaks - incorrect inode logging in unwritten extent conversion - mmap lock vs freeze ordering - remote symlink mishandling - attribute fork removal issues.