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>