mm: fix prctl_set_vma_anon_name

prctl_set_vma_anon_name could attempt to set the name across
two vmas at the same time due to a typo, which might corrupt
the vma list.  Fix it to use tmp instead of end to limit
the name setting to a single vma at a time.

Reported-by: Jed Davis <jld@mozilla.com>
Signed-off-by: Colin Cross <ccross@android.com>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>