BACKPORT: dm bufio: refactor adjust_total_allocated

Refactor adjust_total_allocated() so that it takes a bool argument
indicating if it should add or subtract the buffer size.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
(cherry picked from commit d0a328a385d2d1ab87e7a959d91c1841ed5a498f)

Bug: 136247322
Change-Id: I25522f4991d17c56f709f7c904f4480a427decbf
Signed-off-by: Martijn Coenen <maco@android.com>