rpmsg: char: Avoid double destroy of default endpoint

The rpmsg_dev_remove() in rpmsg_core is the place for releasing
this default endpoint.

So need to avoid destroying the default endpoint in
rpmsg_chrdev_eptdev_destroy(), this should be the same as
rpmsg_eptdev_release(). Otherwise there will be double destroy
issue that ept->refcount report warning:

refcount_t: underflow; use-after-free.

Call trace:
 refcount_warn_saturate+0xf8/0x150
 virtio_rpmsg_destroy_ept+0xd4/0xec
 rpmsg_dev_remove+0x60/0x70

The issue can be reproduced by stopping remoteproc before
closing the /dev/rpmsgX.

Fixes: bea9b79 ("rpmsg: char: Add possibility to use default endpoint of the rpmsg device")
Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Reviewed-by: Arnaud Pouliquen <arnaud.pouliquen@foss.st.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/1663725523-6514-1-git-send-email-shengjiu.wang@nxp.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>

Author: TE-N-ShengjiuWang

drivers/rpmsg/rpmsg_char.c

@@ -76,7 +76,9 @@ int rpmsg_chrdev_eptdev_destroy(struct device *dev, void *data)
 
 	mutex_lock(&eptdev->ept_lock);
 	if (eptdev->ept) {
-		rpmsg_destroy_ept(eptdev->ept);
+		/* The default endpoint is released by the rpmsg core */
+		if (!eptdev->default_ept)
+			rpmsg_destroy_ept(eptdev->ept);
 		eptdev->ept = NULL;
 	}
 	mutex_unlock(&eptdev->ept_lock);