It looks like the new API iov_iter_get_pages is not safe for use
when trying to get a page from a bvec to be sent by kernel_sendpage().
So just revert back our own implentation where we check the page
before making get_page(), if the page can't be sendpage, fall back
to copy mode.
Affects: #PSBM-
Hello!
Ack.
Disgusting at the first sight, but I like this! :-)
Indeed, this iterator is our internal thing, we created it ourselves and
actually we have no reasons to use dubious spoiled system implementation.
One note, formally, BUG_ON is incorrect. We should assert
it->iov_offset + i->bvec->
