libs: update jpnfs to 0.5.1

with following bug fixes and improvements:

- support for old file handles, which will allow to update nfs server without
  remount of existing clients (important for backtort to 2.2).

- reject mount from clients which do not have any export entries.
- respect root user if no_root_squash option is specified in exports file

  [3eb23ef] vfs: fix no_root_squash exports
  [5c12e5d] vfs: support old file handles
  [694da2f] vfs: construct new file handles from old one
  [513d265] test: add unit test for FileHandle and FileHandleBuilder
  [56d803b] nfsv4: consult export file on getRootInode()

Acked-by: Gerd Behrmann
Target: trunk, 2.6
Require-book: no
Require-notes: yes
Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>