torture: Suppress false-positive CONFIG_INITRAMFS_SOURCE complaint

The scripting must supply the CONFIG_INITRAMFS_SOURCE Kconfig option
so that kbuild can find the desired initrd, but the configcheck.sh
script gets confused by this option because it takes a string instead
of the expected y/n/m.  This causes checkconfig.sh to complain about
CONFIG_INITRAMFS_SOURCE in the torture-test output (though not in the
summary).  As more people use rcutorture, the resulting confusion is
an increasing concern.

This commit therefore suppresses this false-positive warning by filtering
CONFIG_INITRAMFS_SOURCE from within the checkconfig.sh script.

Reported-by: Joel Fernandes <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>