ashmem: add mutex to set size ioctl

The ioctl of ashmem is an unlocked method, hence each of its command
functions must have its own locking. Since asma->size is used at different
critical sections, it must remain unchanged until the end of the critical
section that is being executed.

Change-Id: I804604667a25976334c174ba33601bed35df3755
Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>