ksmbd: fix credit count leakage

This patch fix the failure from smb2.credits.single_req_credits_granted
test. When client send 8192 credit request, ksmbd return 8191 credit
granted. ksmbd should give maximum possible credits that must be granted
within the range of not exceeding the max credit to client.

Cc: stable@vger.kernel.org
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>