nfs: fix compatibility with 2.6

post 2.6 nfs code has changes package name from

org.dcache.chimera.nfs => org.dcache.nfs
Nevertheless one class is used inside
a message sent to a pool and brakes
compatibility with.

this change restores compatibility.

Notice, this makes 2.7.6 not compatible in mixed
setup.

Acked-by: Gerd Behrmann
Target: master, 2.7
Require-book: no
Require-notes: no