Use strtobool rather than open coding "on" and "off" parsing.

Link: https://lkml.kernel.org/r/20220227181038.126926-1-linux@treblig.orgSigned-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>