NTB: Skip db_valid validation writing DB link bit

In ntb_poll_link, we are intentionally writing the link bit, which is
absent from db_valid_mask. Don't panic on a kassert when we do so.

The Linux version of this (dual BSD/GPL) driver has the db_valid_mask
assertions in callers of db_iowrite() rather than db_iowrite() itself;
it skips the assertions in the equivalent of ntb_poll_link(). Rather
than duplicating the assertions in every caller, add a db_iowrite_raw()
that doesn't check and use it from ntb_poll_link().

Suggested by: kassert_panic
Sponsored by: EMC / Isilon Storage Division