mon/OSDMonitor: set FLAG_SELFMANAGED_SNAPS on cephfs snap removal

mon/OSDMonitor: set FLAG_SELFMANAGED_SNAPS on cephfs snap removal

CephFS uses a different path to remove selfmanaged snaps than librados,
so while the librados path goes through pg_pool_t::remove_unmanaged_snap(),
we open code the snap addition to the pool's removed_snaps here.  If we
don't set FLAG_SELFMANAGED_SNAPS at that time, we will implicitly set it
during decode and get a CRC mismatch.

Fix by explicitly setting FLAG_SELFMANAGED_SNAPS flag here.

Fixes: http://tracker.ceph.com/issues/23949
Signed-off-by: Sage Weil <sage@redhat.com>