WARN_ON(in_interrupt()) is used for historic reasons to ensure proper usage
of down_read() and predates might_sleep() and lockdep.

down_read() has might_sleep() which also catches users from preemption
disabled regions while in_interrupt() does not.

Remove WARN_ON(in_interrupt()) because there are now better debugging
facilities.

Link: https://lore.kernel.org/r/20210208194400.384003-1-bigeasy@linutronix.deSigned-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>