mm: group pte related helpers together

This reduces/simplifies the diff for the next patch which moves THP
specific code.

No semantical changes !

Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Link: http://lkml.kernel.org/r/1442918096-17454-9-git-send-email-vgupta@synopsys.com


Signed-off-by: Vineet Gupta <vgupta@synopsys.com>