net: tun: rebuild error handling in tun_get_user

[ Upstream commit ab00af85 ]

The error handling in tun_get_user is very scattered.
This patch unifies error handling, reduces duplication of code, and
makes the logic clearer.

Signed-off-by: Chuang Wang <nashuiliang@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: 82b2bc27 ("tun: Fix memory leak for detached NAPI queue.")
Signed-off-by: Sasha Levin <sashal@kernel.org>