ceph: capability management

ceph: capability management

The Ceph metadata servers control client access to data by issuing
capabilities granting clients permission to read and/or write to OSDs
(storage nodes).  Each capability consists of a set of bits indicating
which operations are allowed.

In the case of an EXCL (exclusive) or WR capabilities, the client is
allowed to change inode attributes (e.g., file size, mtime), noting
its dirty state in the ceph_cap, and asynchronously flush that
metadata change to the MDS.

In the event of a conflicting operation (perhaps by another client),
the MDS will revoke the conflicting client capabilities.

In order for a client to cache an inode, it must hold a capability
with at least one MDS server.  When inodes are released, release
notifications are batched and periodically sent en masse to the MDS
cluster to release server state.

Signed-off-by: Sage Weil <sage@newdream.net>