Crash or warning message from cfq when changing blkio cgroup weight values

Issue

• System crashed in cfq_group_service_tree_add
• A system crashed after altering the weight settings of some blkio cgroups.
• A kernel WARNING message similar to below was seen in the logs for function cfq_group_service_tree_del

[74795.195053] WARNING: at block/cfq-iosched.c:1378 cfq_group_service_tree_del+0xd5/0xf0()
[74795.195055] Modules linked in: fuse btrfs zlib_deflate raid6_pq xor vfat msdos fat xfs veth xt_addrtype ipt_MASQUERADE dm_thin_pool dm_persistent_data dm_bio_prison dm_bufio libcrc32c loop dm_mod ip6t_rpfilter ip6t_REJECT ipt_REJECT xt_conntrack ebtable_nat ebtable_broute bridge stp llc ebtable_filter ebtables ip6table_nat nf_conntrack_ipv6 nf_defrag_ipv6 nf_nat_ipv6 ip6table_mangle ip6table_security ip6table_raw ip6table_filter ip6_tables iptable_nat nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4 nf_nat nf_conntrack iptable_mangle iptable_security iptable_raw iptable_filter ip_tables sg coretemp kvm_intel kvm iTCO_wdt iTCO_vendor_support ioatdma i5000_edac serio_raw edac_core e1000e ptp pps_core shpchp lpc_ich pcspkr mfd_core i5k_amb dca i2c_i801 mperf binfmt_misc ext4 mbcache jbd2 sd_mod crc_t10dif
[74795.195126]  crct10dif_common radeon i2c_algo_bit drm_kms_helper ttm ahci libahci drm libata i2c_core
[74795.195138] CPU: 1 PID: 231 Comm: kworker/1:1H Tainted: G          I --------------   3.10.0-123.1.2.el7.x86_64 #1
[74795.195141] Hardware name: Supermicro X7DBU/X7DBU, BIOS 6.00 10/10/2007