client: set metadata["root"] from mount method when it's called with a pathname

client: set metadata["root"] from mount method when it's called with a pathname

Currently, we only set the root properly config file or the
--client_metadata command line option. If a userland client program
tries to call ceph_mount with a pathname, it's not being properly
set.

Since we already hold the mutex, we can just update it directly.

Signed-off-by: Jeff Layton <jlayton@redhat.com>