net: netdevsim: fix nsim_pp_hold_write()

nsim_pp_hold_write() has two problems:

1) It may return with rtnl held, as found by syzbot.

2) Its return value does not propagate an error if any.

Fixes: 1580cbc ("net: netdevsim: add some fake page pool use")
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20241216083703.1859921-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: edumazet

drivers/net/netdevsim/netdev.c

@@ -635,10 +635,10 @@ nsim_pp_hold_write(struct file *file, const char __user *data,
 		page_pool_put_full_page(ns->page->pp, ns->page, false);
 		ns->page = NULL;
 	}
-	rtnl_unlock();
 
 exit:
-	return count;
+	rtnl_unlock();
+	return ret;
 }
 
 static const struct file_operations nsim_pp_hold_fops = {