Fix shared state race in mailbox message handling

When IOCTL calls are handled and when sending messages to the mailbox,
the device mutex is locked. However, when received mailbox messages are
handled, the mutex is not locked so there are possible concurrent access
races in shared states e.g. an inference's status may be updated while
another IOCTL caller is reading it.

To avoid these races, the mutex is now locked while messages received
from the mailbox are handled.

Change-Id: I4d51da542410ab02fb0f890c939269c642176b2c