Add `volatile` specifier for the mapped memory area pointer to give

Add `volatile` specifier for the mapped memory area pointer to give
a hint to the compiler that it cannot optimize any checks to it

Without this hint the compiler tends to optimize the synchronization
barrier as it thinks that if a variable is unchanged in the parent
process, it can optimize access to it and not perform the check each
time during the `while` loop which leads to deadlock (endless waiting).

Change-Id: I0a316b4e0d1546bc6864737c20c86601635fff2a