smb: smbdirect: introduce smbdirect_connection_send_iter() and related functions

smb: smbdirect: introduce smbdirect_connection_send_iter() and related functions

This is basically a copy of the following functions in the server:

smb_direct_post_send       => smbdirect_connection_post_send_wr
smb_direct_send_ctx_init   => smbdirect_connection_send_batch_init
smb_direct_flush_send_list => smbdirect_connection_send_batch_flush
wait_for_send_lcredit      => smbdirect_connection_wait_for_send_lcredit
wait_for_send_credits      => smbdirect_connection_wait_for_send_credits
post_sendmsg               => smbdirect_connection_post_send_io
smb_direct_post_send_data  => smbdirect_connection_send_single_iter
smb_direct_send_iter       => smbdirect_connection_send_iter

They will replace the server functions soon and will also be used in
the client as smbdirect_connection_send_single_iter() is very similar to
smbd_post_send_iter().

There's also a smbdirect_connection_send_wait_zero_pending()
helper that can be used together with
smbdirect_connection_send_single_iter() in the client
until it can use smbdirect_connection_send_iter(),
which can happen with David's refactoring.

Cc: Steve French <smfrench@gmail.com>
Cc: Tom Talpey <tom@talpey.com>
Cc: Long Li <longli@microsoft.com>
Cc: Namjae Jeon <linkinjeon@kernel.org>
Cc: David Howells <dhowells@redhat.com>
Cc: linux-cifs@vger.kernel.org
Cc: samba-technical@lists.samba.org
Signed-off-by: Stefan Metzmacher <metze@samba.org>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>