test/zdtm/mountpoints: clear the umask when creating

 /dev/null

Clear the umask when creating /dev/null and restore the umask
after the creation.
Mountpoints test unlinks and makes new /dev/null inside by mknod.
But in some linux, there is default umask, e.g., as 022. It means that
/dev/null created directly cannot be wrote by group and other users.
It is not reasonable because /dev/null should be accessable to all
users.

Signed-off-by: Yicheng Qin <yichengq@google.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>