mm/sparse: rename pfn_present() to pfn_in_present_section()

After introducing mem sub section concept, pfn_present() loses its literal
meaning, and will not be necessary a truth on partial populated mem
section.

Since all of the callers use it to judge an absent section, it is better
to rename pfn_present() as pfn_in_present_section().

Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>		[powerpc]
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: Leonardo Bras <leonardo@linux.ibm.com>
Cc: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Cc: Nathan Lynch <nathanl@linux.ibm.com>
Link: http://lkml.kernel.org/r/1581919110-29575-1-git-send-email-kernelfans@gmail.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>