sheep/plain_store: move displaced object from read/write operation

This is a defensive patch, but solve a problem we found on the test cluster. The
problem is tricky:

init_vdi_state() errorr out as "failed to read inode header xxx, 0".

After a detailed log analyzing, we found that the cause of this problem is
mismatch of object of path between get_obj_path() and its real path, e.g,

- disk1/oidA (real location)
- disk3/oidA (get_obj_path() location)

So default_read() fails.

To solve this problem and any future problems caused by misplaced object (if any
by any unexpected reason), we call default_exist() before default_{read, write}.
By this means, all the misplaced objects will be placed properly before
any read/write operations.

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
sheep: Correction of two typos.

Signed-off-by: Marcin Mirosław <marcin@mejor.pl>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>