x86/mm: Fix sparse 'tlb_single_page_flush_ceiling' warning and make the variable read-mostly

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: David Rientjes <rientjes@google.com>
Signed-off-by: Jeremiah 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.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/mm/tlb.c

@@ -180,7 +180,7 @@ void flush_tlb_current_task(void)
  *
  * This is in units of pages.
  */
-unsigned long tlb_single_page_flush_ceiling = 33;
+static unsigned long tlb_single_page_flush_ceiling __read_mostly = 33;
 
 void flush_tlb_mm_range(struct mm_struct *mm, unsigned long start,
 				unsigned long end, unsigned long vmflag)