[GFS2] Shrink gfs2_inode (7) - di_payload_format

This is almost never used. Its there for backward
compatibility with GFS1. It doesn't need its own
field since it can always be calculated from the
inode mode & flags. This saves a bit more space
in the gfs2_inode.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>