memshare: Synchronize memory allocation and free operations

Currently, on Modem SSR, memshare driver receives notification
in which it free up the allocated memory. Due to timing issue,
free is happening after allocation. This patch synchronize
these operations.

Change-Id: I464d3b82f33ccdfd0988ef33fec8414d4ab4a550
Signed-off-by: Mohit Aggarwal <maggarwa@codeaurora.org>