iov_iter: Remove last_offset member

iov_iter: Remove last_offset member

With the removal of ITER_PIPE, the last_offset member of struct iov_iter is
no longer used, so remove it and un-unionise the remaining member.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: Matthew Wilcox <willy@infradead.org>
cc: Alexander Viro <viro@zeniv.linux.org.uk>
cc: linux-nfs@vger.kernel.org
cc: linux-fsdevel@vger.kernel.org
cc: linux-mm@kvack.org
cc: netdev@vger.kernel.org
Link: https://lore.kernel.org/r/2933618.1680689716@warthog.procyon.org.uk
Signed-off-by: Jens Axboe <axboe@kernel.dk>