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>