kprobes: Adopt guard() and scoped_guard()

Use guard() or scoped_guard() for critical sections rather than
discrete lock/unlock pairs.

Link: https://lore.kernel.org/all/173289887835.73724.608223217359025939.stgit@devnote2/

Signed-off-by: Masami Hiramatsu (Google) <[email protected]>

Author: mhiramat