ANDROID: mm: page_pinner: remove longterm_pinner

From the experience, longterm_pinner is not worth maintaining
considering how much it churns MM. Just drop the feature and
we are good with alloc_contig_failed.

The visible effect from this patch is
  1. drop $debugfs/page_pinner/longterm_pinner
  2. drop put_user_page expoerted API
  3. rename alloc_contig_failed to buffer

Bug: 218731671
Signed-off-by: Minchan Kim <minchan@google.com>
Change-Id: I68cc11db448260987a9e26b99647ecb55f571616