mm/hmm: select mmu notifier when selecting HMM

To avoid random config build issue, select mmu notifier when HMM is
selected.  In any cases when HMM get selected it will be by users that
will also wants the mmu notifier.

Link: http://lkml.kernel.org/r/20190403193318.16478-2-jglisse@redhat.comSigned-off-by: NJérôme Glisse <jglisse@redhat.com>
Acked-by: NBalbir Singh <bsingharora@gmail.com>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Ira Weiny <ira.weiny@intel.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Souptick Joarder <jrdr.linux@gmail.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/Kconfig

@@ -697,12 +697,12 @@ config DEV_PAGEMAP_OPS
 
 config HMM
 	bool
+	select MMU_NOTIFIER
 	select MIGRATE_VMA_HELPER
 
 config HMM_MIRROR
 	bool "HMM mirror CPU page table into a device page table"
 	depends on ARCH_HAS_HMM
-	select MMU_NOTIFIER
 	select HMM
 	help
 	  Select HMM_MIRROR if you want to mirror range of the CPU page table of a