nsswitch: Fix short writes in winbind_write_sock

We set the socket to nonblocking and don't handle EAGAIN right. We do
a poll anyway, so wait for writability, which should fix this.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=10195
Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
(cherry picked from commit c6909887c26d4e827633acd50b11cf08c6aee0f7)
Signed-off-by: Andreas Schneider <asn@samba.org>