mm: page_alloc: catch out-of-date list of page flag names

String tables with names of enum items are always prone to go out of
sync with the enums themselves.  Ensure during compile time that the
name table of page flags has the same size as the page flags enum.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Gavin Shan <shangw@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>