explain what an inline xattr is and how it relates to omap

The logic of the configuration flags related to xattr is clarified to define what an inline xattr is and when storing in the object map is preferred.

http://tracker.ceph.com/issues/4617 refs #4617

Signed-off-by: Loic Dachary <loic@dachary.org>
client: log inode when waiting on max size

Signed-off-by: Sam Lang <sam.lang@inktank.com>
client: Kick waiters for max size

If the mds restarts without successfully logging a max size
cap update, the client waits indefinitely in Client::get_caps
on the waitfor_caps list.  So when the client gets an mds map
indicating a new active mds has replaced a down mds, we need to
kick the caps update request.  This patch mimics the behavior
in the kernel by setting the wanted_max_size
and requested_max_size to 0 and wakes up the waiters.

Signed-off-by: Sam Lang <sam.lang@inktank.com>