pom: switch to jpnfs 0.5.2

update the nfs library with following fixes:

    [2b13212] nfsv41: consult backend 'filesystem' for read/write access
    [fedb335] nfsv41: propagate layout return errors on close

solves problem with file overwrite
solves problem with delayed checksum calculations on write

Ticket: #7572
Acked-by: Karsten Schwank
Target: master, 2.6
Require-book: no
Require-notes: yes
Merge pull request #142 from kofemann/jpnfs-0.5.2

update jpnfs library for 2.6 branch