msm: adsprpc: double free when creating dynamic process

Adding synchronization access to allocated memory of userPD
to avoid double-freeing the buffer when multiple calls are
made to create dynamic userPD process.

Change-Id: I07b2c290cc400007ce00d96b36c5b40b17ea43c9
Signed-off-by: Edgar Flores <edgarf@codeaurora.org>