alpha: replace bogus in_interrupt()

in_interrupt() is true for a variety of things including bottom half
disabled regions. Deducing hard interrupt context from it is dubious at
best.

Use in_irq() which is true if called in hard interrupt context. Otherwise
calling irq_exit() would do more harm than good.

Link: https://lkml.kernel.org/r/20201113135832.2202833-1-bigeasy@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Tested-by: Serge Belyshev <belyshev@depni.sinp.msu.ru>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Matt Turner <mattst88@gmail.com>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>