jfs: fix readdir cookie incompatibility with NFSv4

NFSv4 reserves readdir cookie values 0-2 for special entries (. and ..),
but jfs allows a value of 2 for a non-special entry. This incompatibility
can result in the nfs client reporting a readdir loop.

This patch doesn't change the value stored internally, but adds one to
the value exposed to the iterate method.

Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>
Tested-by: Christian Kujau <lists@nerdbynature.de>