ceph: ceph_frag_contains_value can be boolean

This patch makes ceph_frag_contains_value return bool to improve
readability due to this particular function only using either one or
zero as its return value.

No functional change.

Signed-off-by: Yaowei Bai <baiyaowei@cmss.chinamobile.com>
Signed-off-by: Yan, Zheng <zyan@redhat.com>