nfs-iostat.py: divide by zero with fresh mount

When an export is freshly mounted, /proc/self/mountstats displays age = 0.
This causes nfs-iostat.py to divide by zero throwing an error.  When we
have age = 0, other stats are greater than 0, so we'll set age = 1 and
print the relevant stats.

Signed-off-by: Kevin Constantine <kevin.constantine@disneyanimation.com>
Signed-off-by: Steve Dickson <steved@redhat.com>