torture: Cause mkinitrd.sh to indicate failure on compile errors

Currently, if the C program created by mkinitrd.sh has compile errors,
the errors are printed, but kvm.sh soldiers on, building kernels that
have init-less initrd setups.  The kernels then fail on boot when they
attempt to mount non-existent root filesystems.

This commit therefore improves user friendliness by making mkinitrd.sh
return non-zero exit status on compile errors, which in turn causes kvm.sh
to take an early exit, with the compile errors still clearly visible.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>