ext4: mark multi-page IO complete on mapping failure

In mpage_da_map_and_submit(), if we have a delayed block
allocation failure from ext4_map_blocks(), we need to mark
the IO as complete, by setting

      mpd->io_done = 1;

Otherwise, we could end up submitting the pages in an outer
loop; since they are unlocked on mapping failure in
ext4_da_block_invalidatepages(), this will cause a bug check
in mpage_da_submit_io().

I tested this by injected failures into ext4_map_blocks().
Without this patch, a simple fsstress run will bug check;
with the patch, it works fine.

Signed-off-by: Curt Wohlgemuth <curtw@google.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>