ceph osd: add support for new op cmpext

ceph osd: add support for new op cmpext
This adds support for a new op cmpext. The request will read
extent.length bytes and compare them to extent.length bytes at
extent.offset on disk. return 0 on success, negative error code
on failure, (-MAX_ERRNO - mismatch_off) on mismatch

rbd will use this in a multi op request to implement the
SCSI COMPARE_AND_WRITE request which is used by VMware for
its atomic test and set request.

Signed-off-by: Zhengyong Wang <wangzhengyong@cmss.chinamobile.com>
Signed-off-by: Mike Christie <mchristi@redhat.com>
[ddiss@suse.de: ReplicatedPG -> PrimaryLogPG]
Reviewed-by: David Disseldorp <ddiss@suse.de>