In find_extend_vma(), we don't need mlock_vma_pages_range() to verify the
vma type - we know we're working with a stack. So, we can call directly
into __mlock_vma_pages_range(), and remove the last make_pages_present()
call site.
Note that we don't use mm_populate() here, so we can't release the mmap_sem
while allocating new stack pages. This is deemed acceptable, because the
stack vmas grow by a bounded number of pages at a time, and these are
anon pages so we don't have to read from disk to populate them.
Signed-off-by: Michel Lespinasse <walken@xxxxxxxxxx>