irqdomain: make irq_linear_revmap() a fast path again

Over the years, irq_linear_revmap() gained tests and checks to make sure
callers were using it safely, which while important, also make it less
of a fast path. After the irqdomain refactoring done recently, it is now
possible to make irq_linear_revmap() a fast path again. This patch moves
irq_linear_revmap() to the header file and makes it a static inline so
that interrupt controller drivers using a linear mapping can decode the
virq from a hwirq in just a couple of instructions.

Signed-off-by: Grant Likely <grant.likely@linaro.org>