mm: pfn_to_section_nr()/section_nr_to_pfn() is valid only in CONFIG_SPARSEMEM context

pfn_to_section_nr()/section_nr_to_pfn() is valid only in CONFIG_SPARSEMEM
context.  Move it to proper place.

Signed-off-by: Daniel Kiper <dkiper@net-space.pl>
Cc: Dave Hansen <dave@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>