ceph: adding protection for showing cap reservation info

Adding spinlock protection during getting cap reservation
ralated fields so that the numbers match below BUG_ON condition
in the code.

BUG_ON(mdsc->caps_total_count != mdsc->caps_use_count +
				 mdsc->caps_reserve_count +
				 mdsc->caps_avail_count);

Signed-off-by: Chengguang Xu <cgxu519@icloud.com>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>