kernel debug: support resetting WARN*_ONCE

kernel debug: support resetting WARN*_ONCE

I like _ONCE warnings because it's guaranteed that they don't flood the
log.

During testing I find it useful to reset the state of the once warnings,
so that I can rerun tests and see if they trigger again, or can
guarantee that a test run always hits the same warnings.

This patch adds a debugfs interface to reset all the _ONCE warnings so
that they appear again:

  echo 1 > /sys/kernel/debug/clear_warn_once

This is implemented by putting all the warning booleans into a special
section, and clearing it.

[[email protected]: coding-style fixes]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Andi Kleen <[email protected]>
Tested-by: Michael Ellerman <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>