ksmbd: validate credit charge after validating SMB2 PDU body size

smb2_validate_credit_charge() accesses fields in the SMB2 PDU body,
but until smb2_calc_size() is called the PDU has not yet been verified
to be large enough to access the PDU dynamic part length field.

Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Ralph Boehme <slow@samba.org>
Signed-off-by: Steve French <stfrench@microsoft.com>