nfs: set file size in namespace on stable_how is FILE_SYNC4

In some workloads, nfs client may request file size before mover
is slosed and file size is set. Nevertheless, prior such requests client
must commit written data by sending an explicit COMMIT operation
or WRITE with FILE_SYNC flag.

This change covers both cases. To match pnfsid's to mover we keep
a dedicated mapping with an assumption, that only single mover can
perform write operation.

2.11 is requested as we see the need for a sync in production system at desy.

Acked-by: Gerd Behrmann
Target: master, 2.11
Require-book: no
Require-notes: yes