generic: add test for direct io partial writes

generic: add test for direct io partial writes

btrfs recently had a bug where a direct io partial write resulted in a
hole in the file. Add a new generic test which creates a 2MiB file,
mmaps it, touches the first byte, then does an O_DIRECT write of the
mmapped buffer into a new file. This should result in the mapped pages
being a mix of in and out of page cache and thus a partial write, for
filesystems using iomap and IOMAP_DIO_PARTIAL.

Signed-off-by: Boris Burkov <boris@bur.io>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Reviewed-by: David Disseldorp <ddiss@suse.de>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>