mon: OSDMonitor: Refuse to delete CephFS pools

To avoid confusing CephFS, don't permit deletion
of pools which are in use as the metadata pool
or any of the data pools.

Signed-off-by: John Spray <john.spray@inktank.com>
mon: MDSMonitor: Forbid removal of first data pool

Because inodes from other pools now store their backtrace
on the first/default data pool, it is special and may not
be removed.

Fixes: 7109

Signed-off-by: John Spray <john.spray@inktank.com>
Merge pull request #1173 from ceph/wip-7109

Fix #7109: Prevent removal of default data pool

Reviewed-by: Greg Farnum <greg@inktank.com>