generic: test page fault during direct IO write with O_APPEND

generic: test page fault during direct IO write with O_APPEND

Test that doing a direct IO append write to a file when the input buffer
was not yet faulted in, does not result in an incorrect file size.

This exercises a bug on btrfs reported by users and which is fixed by
the following kernel patch:

   "btrfs: fix corruption after buffer fault in during direct IO append write"

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>