BACKPORT: dm bufio: call adjust_total_allocated from __link_buffer and __unlink_buffer

Move the call to adjust_total_allocated() to __link_buffer() and
__unlink_buffer() so that only used buffers are counted.  Reserved
buffers are not.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
(cherry picked from commit 26d2ef0cd0f7c12aa331b502c1c1460b85ebd04f)

Bug: 136247322
Change-Id: Ia47e99082a4c1a2acf5f5b5aab1a21fd8b6300df
Signed-off-by: Martijn Coenen <maco@android.com>