A sparse warning is generated about
'tlb_single_page_flush_ceiling' not being declared.

  arch/x86/mm/tlb.c:177:15: warning: symbol
  'tlb_single_page_flush_ceiling' was not declared. Should it be static?

Since it isn't used anywhere outside this file, fix the warning
by making it static.

Also, optimize the use of this variable by adding the
__read_mostly directive, as suggested by David Rientjes.
Suggested-by: NDavid Rientjes <rientjes@google.com>
Signed-off-by: NJeremiah Mahler <jmmahler@gmail.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Mel Gorman <mgorman@suse.de>
Link: http://lkml.kernel.org/r/1407569913-4035-1-git-send-email-jmmahler@gmail.comSigned-off-by: NIngo Molnar <mingo@kernel.org>