mmu: fix how mappings into ARC buffer are tracked

Currently all mapping are keyed on ARC buffer start when mapping is
added, but on remove pointer into ARC buffer is used, so remove may
leave no longer valid mappings in the database. This patch fixes it
by using a pointer into ARC as a key, the same pointer that is used
during removal.

Signed-off-by: Gleb Natapov <gleb@cloudius-systems.com>
Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>