ksmbd: move credit charge deduction under processing request

Moves the credit charge deduction from total_credits under the processing
a request. When repeating smb2 lock request and other command request,
there will be a problem that ->total_credits does not decrease.

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