ceph: change variable name to follow common rule

Variable name ci is mostly used for ceph_inode_info.
Variable name fi is mostly used for ceph_file_info.
Variable name cf is mostly used for ceph_cap_flush.

Change variable name to follow above common rules
in case of confusing.

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