fix use-after-free bug in fp_final_exit

percpu G(ks)->stats variable is freed in exit_stats,
but will use in kp_free(ks, ks), this will cause
memory corruption bug.

Based on nr_mem_free variable in G(ks)->stats don't
give too much debug info to user, so just kill it,
to fix this use-after-free bug in simply way.

This bug was found when use 'prove -j4 test'.

Reported-by: Yichun Zhang <agentzh@gmail.com>
Signed-off-by: Jovi Zhangwei <jovi.zhangwei@gmail.com>