tools: rcu: Add usage function and check for argument

This commit converts extract-stall.sh script's header comment to a
usage() function, and adds an argument check.  While in the area, make
this script be executable.

[ paulmck: Strength argument check, remove extraneous comment. ]

Signed-off-by: Bhaskar Chowdhury <unixbhaskar@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>