smb: smbdirect: introduce smbdirect_socket_wait_for_credits()

smb: smbdirect: introduce smbdirect_socket_wait_for_credits()

This is a copy of wait_for_credits() in the server, which
will be replaced by this soon.

This will allow us to share more common code between client
and server soon.

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: 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>