mon: OSDMonitor: do not allow changing an erasure-coded pool's size

Fixes: 7277

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
Merge pull request #1 from ceph/wip-7277.for-loic

mon: OSDMonitor: do not allow changing an erasure-coded pool's size

Reviewed-by: Loic Dachary <loic@dachary.org>