libcli/smb: add max_credits arg to smbXcli_negprot_send()

libcli/smb: add max_credits arg to smbXcli_negprot_send()

This allows source4/torture code to set the option for tests by
preparing a struct smbcli_options with max_credits set to some value and
pass that to a torture_smb2_connection_ext().

This will be used in subsequent smbtorture test for SMB2 creditting.

Behaviour of existing upper layers is unchanged, they simply pass the
wanted max credits value to smbXcli_negprot_send() instead of
retrofitting it with a call to smb2cli_conn_set_max_credits().

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>