rcutorture/kvm-recheck-*: Improve result directory readability check

The kvm-recheck-(lock|rcu|rcuperf).sh scripts check whether the
user-specified results directory exists.  If not, it prints out error
message that says the specified directory is unreadable.  To make the
message more precise, this commit adds a readability check.

Fixes: 2193e160 ("rcutorture: Abstract kvm-recheck.sh")

Signed-off-by: SeongJae Park <sj38.park@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>