Allow concurrent calls to unmap buffer by reducing the
mutex lock duration, and allow same fd for mapping user iova,
use book keeping for user smmu.
CRs-Fixed: 2559879
Change-Id: Ic3c92d4fa0ca7d1c29e621172e0b058420e1501d
Signed-off-by: Haibin Liu <haibinl@codeaurora.org>