small fixes and optimizations for delalloc and reflink

[Christoph Hellwig <hch@xxxxxx>]

Hi all,

this series contains my changes to fix up the delalloc and reflink
code to prepare for the always COW mode.  This sits on top of
the 'xfs: properly invalidate cached writeback mapping' series from
Brian.

To make reviewing easier I also have a git tree available here:

    git://git.infradead.org/users/hch/xfs.git xfs-mapping-validation.4

Gitweb:

    http://git.infradead.org/users/hch/xfs.git/shortlog/refs/heads/xfs-mapping-validation.4