fixup: separate vma->lock from vm_area_struct

fixup: separate vma->lock from vm_area_struct

vma->lock being part of the vm_area_struct causes performance regression
during page faults because during contention it's count and owner fields
are constantly updated and having other parts of vm_area_struct used
during page fault handling next to them causes constant cache line
bouncing. Fix that by moving the lock outside of the vm_area_struct.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>