sheepfs: add missing commas to help message

This is necessary to autogenerate the sheepfs manual.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
man: autogenerate sheepdog manuals

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
sheep: don't use journal for epoch write

It doesn't often happen to write data to epoch files.  Let's use
atomic_create_write() for it since it is much simpler than journaling.

This can also removes check_epoch(), which wrongly removes epoch files
after we change the epoch format size.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
migrate: add helper to calculate file size

This prepares for the migration from v1 to v2.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
migrate: add helper to do process for each epoch

This prepares for the migration from v1 to v2.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
migrate: add support for upgrading from v0.5.5 or earlier

Currently, running sheeepdog on the backend store which was formated
by v0.5.5 or earlier breaks epoch information.  This adds support for
the migration.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
sheep: check store format at compile time

This tries to detect unintentional break of the backend format.  Of
course, this check is not enough, but much better than doing nothing.

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