generic: test mapped write after shutdown and failed writeback

generic: test mapped write after shutdown and failed writeback

XFS has a regression where it failed to check shutdown status in the
write fault path. This produced an iomap warning if the page
happened to recently fail a writeback attempt because writeback
failure can clear Uptodate status on the page.

Add a test for this scenario to help ensure mapped write failures
are handled as expected in the event of filesystem shutdown.
Upstream commit e4826691cc7e ("xfs: restore shutdown check in mapped
write fault path") fixed this bug.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>