cifs: add server->vals->header_preamble_size

This variable is set to 4 for all protocol versions and replaces
the hardcoded constant 4 throughought the code.
This will later be updated to reflect whether a response packet
has a 4 byte length preamble or not once we start removing this
field from the SMB2+ dialects.

Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>