um: protect VMA iteration

Due to changes in the iteration, there are now lockdep
checks indicating that we're missing locking here. Add
the missing locking where it's needed.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-By: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>