client: populate metadata during mount

client: populate metadata during mount

This way we avoid having to over-write the "root"
metadata during mount, and any user-set overrides (such
as bad values injected by tests) will survive.

Because Client instances may also open sessions without
mounting to send commands, add a call into populate_metadata
from mds_command as well.

Fixes: http://tracker.ceph.com/issues/18361
Signed-off-by: John Spray <john.spray@redhat.com>
(cherry picked from commit 1dbff09ad553f9ff07f4f4217ba7ece6c2cdc5d2)