filestore: create snap_0 on mkfs

filestore: create snap_0 on mkfs

If we create a new filestore, apply one transaction, and then crash, we
want to make sure roll back to a consistent reference point--empty.  The
simplest solution is to create that snap_0 during mkfs.  This avoids
strangeness like

2012-02-27 00:42:00.336703 7fb1381ef780 filestore(/ceph/osd.0) mkfs in /ceph/osd.0
2012-02-27 00:42:00.341399 7fb1381ef780 journal _open /ceph/osd.0.journal fd 10: 1048576000 bytes, block size 4096 bytes, directio = 1, aio = 0
2012-02-27 00:42:00.349705 7fb1381ef780 filestore(/ceph/osd.0) mkjournal created journal on /ceph/osd.0.journal
2012-02-27 00:42:00.349728 7fb1381ef780 filestore(/ceph/osd.0) mkfs done in /ceph/osd.0
2012-02-27 00:42:00.349787 7fb1381ef780 filestore(/ceph/osd.0) mount FIEMAP ioctl is NOT supported
2012-02-27 00:42:00.349800 7fb1381ef780 filestore(/ceph/osd.0) mount detected btrfs
2012-02-27 00:42:00.349813 7fb1381ef780 filestore(/ceph/osd.0) mount btrfs CLONE_RANGE ioctl is supported
2012-02-27 00:42:00.357023 7fb1381ef780 filestore(/ceph/osd.0) mount btrfs SNAP_CREATE is supported
2012-02-27 00:42:00.405174 7fb1381ef780 filestore(/ceph/osd.0) mount btrfs SNAP_DESTROY is supported
2012-02-27 00:42:00.405214 7fb1381ef780 filestore(/ceph/osd.0) mount btrfs START_SYNC got (25) Inappropriate ioctl for device
2012-02-27 00:42:00.405228 7fb1381ef780 filestore(/ceph/osd.0) mount btrfs START_SYNC is NOT supported: (25) Inappropriate ioctl for device
2012-02-27 00:42:00.405235 7fb1381ef780 filestore(/ceph/osd.0) mount WARNING: btrfs snaps enabled, but no SNAP_CREATE_V2 ioctl (from kernel 2.6.37+)
2012-02-27 00:42:00.405561 7fb1381ef780 filestore(/ceph/osd.0) mount found snaps <>
2012-02-27 00:42:00.405576 7fb1381ef780 filestore(/ceph/osd.0) mount WARNING: no consistent snaps found, store may be in inconsistent state

and subsequent badness if we fail before a proper commit is made.

Fixes: #2105
Signed-off-by: Sage Weil <sage.weil@dreamhost.com>