Documentation: in_irq() cleanup

Replace the obsolete and ambiguos macro in_irq() with new
macro in_hardirq().

Signed-off-by: Changbin Du <changbin.du@gmail.com>

Link: https://lore.kernel.org/r/20210814014831.53083-1-changbin.du@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>