Revise rule for inode data blocks as leafs.

Revise rule for inode data blocks as leafs.

We cannot process an inode data block as a leaf before processing
the InoIdx block.

Previously we would unpin an inode data block if the InoIdx block
should take priority.  But that is problematic.
Instead we simply take the inode data block off the leaf list.

This means we have to put it back on when the InoIdx gets unpinned
or phase-flipped.

At same time, tidy up determination of 'is a leaf' as this is used
both when adding something to a leaf list, and when taking something
off.

Signed-off-by: NeilBrown <neilb@suse.de>