Kernel panic with "Bug unable to handle kernel NULL pointer" but kdump failed to collect vmcore.

Issue

• Two systems crashed recently with "Bug unable to handle kernel NULL pointer" but kdump couldn't collect the vmcore.
• Kdump failed with following messages:

..
Loading mbcache.ko module
Loading jbd2.ko module
Loading crc-t10dif.ko module
Loading scsi_tgt.ko module
Loading cdrom.ko module
Loading iomemory-vsl.ko module
INFO: task insmod:277 blocked for more than 120 seconds.
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
INFO: task insmod:277 blocked for more than 120 seconds.
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.