COW improvements and always_cow support V3

[Christoph Hellwig <hch@lst.de>]

Hi all,

this series adds the always_cow mode support after improving our COW
write support a little bit first.

The always_cow mode stresses the COW path a lot, but with a few xfstests
fixups it generall looks good, except for a few tests that complain about
fragmentation, which is rather inherent in this mode, and xfs/326 which
inserts error tags into the COW path not getting the expected result.

Changes since v2:
 - add a patch to remove xfs_trim_extent_eof
 - add a patch to remove the separate io_type and rely on existing state
   in the writeback path
 - rework the truncate race handling in the writeback path a little more

Changes since v1:
 - make delalloc and unwritten extent conversions simpler and more robust
 - add a few additional cleanups
 - support all fallocate modes but actual preallocation
 - rebase on top of a fix from Brian (which is included as first patch
   to make the patch set more usable)