libs: use bugfix release of jpnfs

chagelog:
  * [adfcb19] nfs: fix readdir caching

Ticket: #7978
Acked-by: Paul Millar
Target: master, 2.6
Require-book: no
Require-notes: yes
(cherry picked from commit a15706f84fad25d13dcdcf68d1674472925d3e3a)
Merge pull request #253 from kofemann/new-jpnfs-2.6

libs: use bugfix release of jpnfs