Maps do not free all memory when elements get deleted
( https://github.com/golang/go/issues/20135 ).

As a workaround, we recreate our two big maps (kernelNodeIds & stableAttrs)
when they have shrunk dramatically (100 x smaller).

Benchmarkung with loopback (go version go1.16.2 linux/amd64) shows:

Before this change:

Step               VmRSS (kiB)
-----              -----------
Fresh mount          4000
ls -R 500k files   271100
after drop_cache   336448
wait ~ 3 minutes   101588

After:

Step               VmRSS (kiB)
-----              -----------
Fresh mount          4012
ls -R 500k files   271100
after drop_cache    31528

Results for gocryptfs are similar.

Has survived xfstests via gocryptfs.

Fixes: https://github.com/rfjakob/gocryptfs/issues/569
Change-Id: Idcae1ab953270516735839a034d586717647b8db