os/FileStore: fix journal-less operation

A few different fixes here:

- make the op_seq correct on start when there is no replay
- make an explicit 'no journal' case in queue_transaction (one that
  actually works).  Note that the 'trailing journal' mode is probably
  still broken.
- add a -J flag to vstart.sh for journalless operation

Signed-off-by: Sage Weil <sage@inktank.com>
os/FileStore: refactor backend instantiation

Use a factory method for creating the backend so that the logic is not
duplicated in mkfs() and _detect_fs().  Make the other fs-dependent logic
(xattr limits) key off of the fs_type magic.

Signed-off-by: Sage Weil <sage@inktank.com>
os/FileStore: add f2fs backend (incomplete)

Signed-off-by: Sage Weil <sage@inktank.com>