mm: select HAVE_MOVE_PMD on x86 for faster mremap

Moving page-tables at the PMD-level on x86 is known to be safe.  Enable
this option so that we can do fast mremap when possible.

Link: http://lkml.kernel.org/r/20181108181201.88826-4-joelaf@google.com
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Suggested-by: Kirill A. Shutemov <kirill@shutemov.name>
Acked-by: Kirill A. Shutemov <kirill@shutemov.name>
Cc: Julia Lawall <Julia.Lawall@lip6.fr>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: William Kucharski <william.kucharski@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/x86/Kconfig

@@ -172,6 +172,7 @@ config X86
 	select HAVE_MEMBLOCK_NODE_MAP
 	select HAVE_MIXED_BREAKPOINTS_REGS
 	select HAVE_MOD_ARCH_SPECIFIC
+	select HAVE_MOVE_PMD
 	select HAVE_NMI
 	select HAVE_OPROFILE
 	select HAVE_OPTPROBES