mknod/mknod07.c: cleanup

Delete some useless comment.
Move the test body from main() to mknod_verify().
Some cleanup.

Signed-off-by: Xiaoguang Wang <wangxg.fnst@cn.fujitsu.com>
Signed-off-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>
mknod/mknod07.c: add EROFS and ELOOP error value tests

Signed-off-by: Xiaoguang Wang <wangxg.fnst@cn.fujitsu.com>
Signed-off-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>