LU-12355 llite: Lustre specific iov_for_each broken (removed)

LU-12355 llite: Lustre specific iov_for_each broken (removed)

Kernel 4.20 introduced iov_iter_type and broke iov_for_each

As iov_for_each is only used once so drop the macro entirely.
When iov_iter_type is available ignore invalid iter types.

Linux-commit: 8a363970d1dc38c4ec4ad575c862f776f468d057

Kernel 3.15 added type to iov_iter. Use the type to provide
a sensible replacement for iov_iter_type when it is available.

Linux-commit: 71d8e532b1549a478e6a6a8a44f309d050294d00

This patch is back-ported from the following one:
Lustre-commit: d93aa0171a25f8ffca51bed35a2d477a45fda0f3
Lustre-change: https://review.whamcloud.com/35024

Cray-bug-id: LUS-6962
Change-Id: I97cdce1c85803ac2d4436d4eedf67a545ea2cdb8
Signed-off-by: Shaun Tancheff <stancheff@cray.com>
Reviewed-on: https://review.whamcloud.com/38479
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>