ion: msm: Add support for dynamically preventing dma-buf mappings

The current rules for determining if a buffer can or cannot be mapped
to depend on how a buffer was allocated. This is restrictive
in environments where the security state of a buffer can change
dynamically, as the accessibility of the buffer has to change with
respect to the security state of the buffer.

Thus, add support to track the number of userspace mappings associated
with an ION buffer, as well as an interface to allow drivers to lock
and unlock a buffer. In this context, locking a buffer means that the
buffer will no longer be mappable, and the buffer does not
have any outstanding mappings at the time the buffer is locked.
Unlocking the buffer means that the buffer can be mapped again.

Change-Id: I6aa73b9ac7c301b12106ad3d3bcb4c2aac959e55
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>