mm: page_alloc: validate buddy before check its migratetype.

Whenever a buddy page is found, page_is_buddy() should be called to
check its validity.  Add the missing check during pageblock merge check.

Fixes: 1dd214b8

 ("mm: page_alloc: avoid merging non-fallbackable pageblocks with others")
Link: https://lore.kernel.org/all/20220330154208.71aca532@gandalf.local.home/
Reported-and-tested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Zi Yan <ziy@nvidia.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>