nfsv4: use concurrent hash map for mover bookkeeping

Acked-by: Gerd Behrmann
Require-notes: no
Require-book: no
target: master, 2.6
(cherry picked from commit aeb2f49c4dcc9d86e953870491501731c125c66d)
Merge pull request #222 from kofemann/nfs-race-2.6

nfsv4: use concurrent hash map for mover bookkeeping