dm: Make rt aware

Use the BUG_ON_NORT variant for the irq_disabled() checks. RT has
interrupts legitimately enabled here as we cant deadlock against the
irq thread due to the "sleeping spinlocks" conversion.

Reported-by: Luis Claudio R. Goncalves <lclaudio@uudg.org>

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>