rcutorture: Include grace-period sequence numbers in failure/close-call

rcutorture: Include grace-period sequence numbers in failure/close-call

This commit includes the grace-period sequence numbers at the beginning
and end of each segment in the "Failure/close-call rcutorture reader
segments" list.  These are in hexadecimal, and only the bottom byte.
Currently, only RCU is supported, with its three sequence numbers (normal,
expedited, and polled).

Note that if all the grace-period sequence numbers remain the same across
a given reader segment, only one copy of the number will be printed.
Of course, if there is a change, both sets of values will be printed.

Because the overhead of collecting this information can suppress
heisenbugs, this information is collected and printed only in kernels
built with CONFIG_RCU_TORTURE_TEST_LOG_GP=y.

[ paulmck: Apply Nathan Chancellor feedback for IS_ENABLED(). ]
[ paulmck: Apply feedback from kernel test robot. ]

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Tested-by: kernel test robot <oliver.sang@intel.com>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>