Protect FFA sync_send_receive with a single mutex

- In sync_send_receive only support access from a single
    thread at a time.  Add the mutex to protect multi-instance

  - This patch is imported from
    https://p4sw-swarm.nvidia.com/changes/32944487

Bug 4135491
Change-Id: Icdcce1df570639e580fbe238ee6260fd2178ef20
Signed-off-by: Yang Shi <yashi@nvidia.com>
Reviewed-on: https://git-master.nvidia.com/r/c/linux-nv-oot/+/2920915
Reviewed-by: Bharat Nihalani <bnihalani@nvidia.com>
Reviewed-by: Chris Dragan <kdragan@nvidia.com>
GVS: Gerrit_Virtual_Submit <buildbot_gerritrpt@nvidia.com>

drivers/misc/mods/mods_arm_ffa.c

@@ -26,6 +26,8 @@ struct mods_ffa_ctx {
 #endif
 };
 
+static DEFINE_MUTEX(mods_ffa_lock);
+
 static struct mods_ffa_ctx mods_ffa_info;
 
 static int ffa_probe(struct ffa_device *ffa_dev)
@@ -137,7 +139,9 @@ int esc_mods_arm_ffa_cmd(struct mods_client *client,
 		return err;
 	}
 
+	mutex_lock(&mods_ffa_lock);
 	err = mods_ffa_info.ffa_ops->sync_send_receive(mods_ffa_info.ffa_dev, &data);
+	mutex_unlock(&mods_ffa_lock);
 
 	switch (p->cmd) {
 	case MODS_FFA_CMD_READ_REG: