rtac: Add mutex lock to ensure proper fops access

Add mutex lock in rtac_open and rtac_release
to avoid usage count discrepancies leading
to multiple calls to unmap memory resulting in
null pointer dereference.

CRs-Fixed: 2271712
Change-Id: Ie6da28837c352030b8d7e377d68a70cf04e7072a
Signed-off-by: Tanya Dixit <tdixit@codeaurora.org>