irq/core: Fix boot crash when the irqaffinity= boot parameter is passed on CPUMASK_OFFSTACK=y kernels(v1)

When the irqaffinity= kernel parameter is passed in a CPUMASK_OFFSTACK=y
kernel, it fails to boot, because zalloc_cpumask_var() cannot be used before
initializing the slab allocator to allocate a cpumask.

So, use alloc_bootmem_cpumask_var() instead.

Also do some cleanups while at it: in init_irq_default_affinity() remove
an #ifdef via using cpumask_available().
Signed-off-by: Rakib Mullick <rakib.mullick@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20171026045800.27087-1-rakib.mullick@gmail.com
Link: http://lkml.kernel.org/r/20171101041451.12581-1-rakib.mullick@gmail.comSigned-off-by: Ingo Molnar <mingo@kernel.org>