client: defer swapping the newmap to mdsmap

client: defer swapping the newmap to mdsmap

This is preparing for adding the command lock support.

For the new mds map we could access it without the client_lock, so
before swaping it to the `mdsmap` member, it will be safe to access.

With this we can defer to hold the client_lock and it will be safe
for the new command_lock to avoid possible dead lock in future or
something else.

Fixes: https://tracker.ceph.com/issues/46620
Signed-off-by: Xiubo Li <xiubli@redhat.com>