mm/mmu_notifier: init notifier if necessary

mm/mmu_notifier: init notifier if necessary

While registering MMU notifier, new instance of MMU notifier_mm will be
allocated and later free'd if currrent mm_struct's MMU notifier_mm has
been initialized.  That causes some overhead.  The patch tries to
elominate that.

Signed-off-by: Gavin Shan <[email protected]>
Signed-off-by: Wanpeng Li <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Avi Kivity <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Marcelo Tosatti <[email protected]>
Cc: Xiao Guangrong <[email protected]>
Cc: Sagi Grimberg <[email protected]>
Cc: Haggai Eran <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>