move the extent count and format into struct xfs_ifork v2

[Christoph Hellwig <hch@xxxxxx>]

Hi all,

this series moves the extent count and format fields into the xfs_ifork
structure.  It is based on the "dinode reading cleanups v2" series.

Git tree:

    git://git.infradead.org/users/hch/xfs.git xfs-ifork-cleanup.2

Gitweb:

    http://git.infradead.org/users/hch/xfs.git/shortlog/refs/heads/xfs-ifork-cleanup.2

Changes since v1:
 - pick up a different scrub prep patch from Darrick
 - keep the nextents == 0 assert in xfs_attr_fork_remove
 - keep clearing various ifork fields in xfs_ifork_destroy
 - add more comments