generic: test reporting of wb errors via syncfs

generic: test reporting of wb errors via syncfs

Add a test for new syncfs error reporting behavior. When an inode fails
to be written back, ensure that a subsequent call to syncfs() will also
report an error.

Kernel with the following patches should pass the test:

  vfs: track per-sb writeback errors and report them to syncfs
  buffer: record blockdev write errors in super_block that it backs

Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>