mds: save the metadata pool id MDSRank class's private member

mds: save the metadata pool id MDSRank class's private member

There is one rare case that when mds daemon received a new mdsmap
and during decoding it, the metadata_pool will be reset to -1, if
other threads try to get it from old mdsmap it will get a invalid
pool id.

This can also help get rid of the mds_lock else where.

Fixes: https://tracker.ceph.com/issues/50389
Signed-off-by: Xiubo Li <xiubli@redhat.com>
Signed-off-by: Xiubo Li <xiubli@redhat.com>