mm: use __pfn_to_section() instead of open coding it

It is defined in the same file just a few lines above.

Link: https://lkml.kernel.org/r/4598487.Rc0NezkW7i@mobilepool36.emlix.com
Signed-off-by: Rolf Eike Beer <eb@emlix.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>