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.

This behavior is currently dependent on this patchset being merged:

    vfs: have syncfs() return error when there are writeback errors

Signed-off-by: Jeff Layton <jlayton@kernel.org>