lafs_refile: factor out check for unpinnable data blocks.

lafs_refile: factor out check for unpinnable data blocks.

Data blocks can be unpinned while refcounts are still present.

Factor that case out into a stand-alone test.

We still test for datablocks being unpinnable in the 'refcnt==0'
case as PinPending might have only just been cleared.

This leaves a large bulk of tests all of which depend on
refcnt reaching zero which can be improved in a subsequent patch.

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