ceph: handle full condition by cancelling ops

Signed-off-by: John Spray <john.spray@redhat.com>
HACK: don't populate statfs from mon

 - to get around the ENOSPC check
 that VFS does on our behalf before
 we even see the osd map.
ceph: implement ceph_osdc_wait_for_map

For use in the filesystem client when receiving
capabilities whose use is contigent on having
a particular OSD map.

Signed-off-by: John Spray <john.spray@redhat.com>
ceph: update ceph_msg_header structure

2 bytes of what was reserved space is now used by
userspace for the compat_version field.

Signed-off-by: John Spray <john.spray@redhat.com>
ceph: update CAPRELEASE message format

Version 2 includes the new osd epoch barrier
field.

Signed-off-by: John Spray <john.spray@redhat.com>
ceph: handle new osdmap barrier field in CLIENTCAPS

Signed-off-by: John Spray <john.spray@redhat.com>