add ffi sample: samples/ffi/ffi_kmalloc.kp

Use ffi to make kmalloc and kfree call.

Signed-off-by: Jovi Zhangwei <jovi.zhangwei@gmail.com>
use vfprintf in usage()

Signed-off-by: Qingping Hou <qingping.hou@gmail.com>
print ktap version on -V option

Signed-off-by: Qingping Hou <qingping.hou@gmail.com>