inet: factor out locked section of inet_accept() in a new helper

[ Upstream commit 711bdd51

 ]

No functional changes intended. The new helper will be used
by the MPTCP protocol in the next patch to avoid duplicating
a few LoC.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Stable-dep-of: 26afda78

 ("net: relax socket state check at accept time.")
Signed-off-by: Sasha Levin <sashal@kernel.org>