mm: compaction: validate pfn range passed to isolate_freepages_block

Commit 0bf380bc ("mm: compaction: check pfn_valid when entering a
new MAX_ORDER_NR_PAGES block during isolation for migration") added a
check for pfn_valid() when isolating pages for migration as the scanner
does not necessarily start pageblock-aligned.

Since commit c89511ab

 ("mm: compaction: Restart compaction from near
where it left off"), the free scanner has the same problem.  This patch
makes sure that the pfn range passed to isolate_freepages_block() is
within the same block so that pfn_valid() checks are unnecessary.

In answer to Henrik's wondering why others have not reported this:
reproducing this requires a large enough hole with the right aligment to
have compaction walk into a PFN range with no memmap.  Size and
alignment depends in the memory model - 4M for FLATMEM and 128M for
SPARSEMEM on x86.  It needs a "lucky" machine.

Reported-by: Henrik Rydberg <rydberg@euromail.se>
Signed-off-by: Mel Gorman <mgorman@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>