Break linkage between inode and dblock at earliest opportunity.

Break linkage between inode and dblock at earliest opportunity.

clear_inode is the first chance to break this linkage,
so do it there.
It is still possible for lafs_iget to get a new inode before
clear_inode has completed, so we need to do the same
test/clear in lafs_iget if b->my_inode is found to be non-NULL;

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