svcgss: move init code into separate function

We've let svcauth_gss_accept() get much too long and hairy.  The
RPC_GSS_PROC_INIT and RPC_GSS_PROC_CONTINUE_INIT cases share very little
with the other cases, so it's very natural to split them off into a
separate function.

This will also nicely isolate the piece of code we need to parametrize
to authenticating gss-protected NFSv4 callbacks on behalf of the NFS
client.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Acked-by: Neil Brown <neilb@suse.de>