ceph: add missing locking to protect i_snap_realm_item during split

All ci->i_snap_realm_item/realm->inodes_with_caps manipulation should be
protected by realm->inodes_with_caps_lock.  This bug would have only bit
us in a rare race with a realm split (during some snap creations).

Signed-off-by: Sage Weil <sage@newdream.net>