mm/page_alloc.c: extract the common part in pfn_to_bitidx()

The return value calculation is the same both for SPARSEMEM or not.

Just take it out.

Signed-off-by: Wei Yang <richard.weiyang@linux.alibaba.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Mel Gorman <mgorman@suse.de>
Link: http://lkml.kernel.org/r/20200623124201.8199-2-richard.weiyang@linux.alibaba.com


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>