rpmsg: char: treat rpmsg_trysend() ENOMEM as EAGAIN

rpmsg_trysend() returns -ENOMEM when no rpmsg buffer can be allocated.
this causes write to fail with this error as opposed to -EAGAIN.
this is what user space applications (and libraries like boost.asio)
would expect when using normal character devices.
Signed-off-by: Tim Blechmann <tim@klingt.org>
Cc: Arnaud Pouliquen <arnaud.pouliquen@foss.st.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20220313024541.1579848-2-tim@klingt.org

drivers/rpmsg/rpmsg_char.c

@@ -242,10 +242,13 @@ static ssize_t rpmsg_eptdev_write_iter(struct kiocb *iocb,
 		goto unlock_eptdev;
 	}
 
-	if (filp->f_flags & O_NONBLOCK)
+	if (filp->f_flags & O_NONBLOCK) {
 		ret = rpmsg_trysendto(eptdev->ept, kbuf, len, eptdev->chinfo.dst);
-	else
+		if (ret == -ENOMEM)
+			ret = -EAGAIN;
+	} else {
 		ret = rpmsg_sendto(eptdev->ept, kbuf, len, eptdev->chinfo.dst);
+	}
 
 unlock_eptdev:
 	mutex_unlock(&eptdev->ept_lock);