Revert "sysctl: clean up nr_pdflush_threads leftover"

This reverts commit 77cab92a.

Heiko reports that this breaks building of strace, which isn't ok.

Reported-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Rafael Aquini <aquini@redhat.com>
Cc: William Kucharski <william.kucharski@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>