ceph: refactor readpage_nounlock() to make the logic clearer

If the return value of ceph_osdc_readpages() is not negative,
it is certainly greater than or equal to zero.

Remove the useless condition judgment and redundant braces.

Signed-off-by: Zhang Zhen <zhenzhang.zhang@huawei.com>
Reviewed-by: Yan, Zheng <zheng.z.yan@intel.com>