iomap infrastructure and multipage writes V3

[Christoph Hellwig]

This series add a new file system I/O path that uses the iomap structure
introduced for the pNFS support and support multi-page buffered writes.

This was first started by Dave Chinner a long time ago, then I did beat
it into shape for production runs in a very constrained ARM NAS
enviroment for Tuxera almost as long ago, and now half a dozen rewrites
later it's back.

The basic idea is to avoid the per-block get_blocks overhead
and make use of extents in the buffered write path by iterating over
them instead.

This also includes the series to take full advantage of the iomap
infrastructure for block zeroing now.

Changes since V2:
 - fix the range for delalloc punches after failed writes
 - updated some changelogs

Chances since V1:
 - add support for fiemap
 - fix a test fail on 1k block sizes
 - prepare for 64-bit length, this will be used in a follow on patchset