rcutorture: Automatically create initrd directory

The rcutorture scripts currently expect the user to create the
tools/testing/selftests/rcutorture/initrd directory.  Should the user
fail to do this, the kernel build will fail with obscure and confusing
error messages.  This commit therefore adds explicit checks for the
tools/testing/selftests/rcutorture/initrd directory, and if not present,
creates one on systems on which dracut is installed.  If this directory
could not be created, a less obscure error message is emitted and the
test is aborted.

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Connor Shu <Connor.Shu@ibm.com>
[ paulmck: Adapt the script to fit into the rcutorture framework and
  severely abbreviate the initrd/init script. ]
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>