gssd: Clean up gssd_setup_krb5_user_gss_ccache()

gssd: Clean up gssd_setup_krb5_user_gss_ccache()

Remove a contradictory portion of the block comment documenting
gssd_find_existing_krb5_ccache().  This should have been removed by
commit 289ad31e, which reversed the meaning of the function's return
values.

Note that, in user space, typically errno's are positive.  But here
we follow the kernel convention of using negative values to return
error codes.  Make the documenting comments explicit about the sign
of an error return -- it will never be positive in the case of an
error.

And a nit: At the last return statement in
gssd_setup_krb5_user_gss_ccache(), "err" always contains zero, as
far as I can tell.  Make it explicit (to human readers) that when
execution reaches this point, gssd_setup_krb5_user_gss_ccache() is
going to return "success."

Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>