xfs: xfs inode structure cleanups

This series (from Christoph Hellwig) cleans up how we define struct
xfs_inode.  Most importantly it removes the pointles xfs_icdinode
sub-structure.

Changes since v2:
 - rebased to 5.12-rc
 - remove the legacy DMAPI fields entirely
 - ensure freshly allocated inodes have diflags2 cleared

Changes since v1:
 - rebased to 5.8-rc
 - use xfs_extlen_t fo i_cowextsize
 - cleanup xfs_fill_fsxattr