vti: Simplify error handling in module init and exit

The error handling in the module init and exit functions can be
shortened to safe us some code.

1/ Remove the code duplications in the init function, jump straight to
the existing cleanup code by adding some labels. Also give the error
message some more value by telling the reason why loading the module has
failed. Furthermore fix the "IPSec" typo -- it should be "IPsec" instead.

2/ Remove the error handling in the exit function as the only legitimate
reason xfrm4_protocol_deregister() might fail is inet_del_protocol()
returning -1. That, in turn, means some other protocol handler had been
registered for this very protocol in the meantime. But that essentially
means we haven't been handling that protocol any more, anyway. What it
definitely means not is that we "can't deregister tunnel". Therefore
just get rid of that bogus warning. It's plain wrong.

Signed-off-by: Mathias Krause <minipli@googlemail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>