ksmbd: set NegotiateContextCount once instead of every inc

There are no early returns, so marshalling the incremented
NegotiateContextCount with every context is unnecessary.

Signed-off-by: David Disseldorp <ddiss@suse.de>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>