Kernel panic in cow_read_mapping

[lishuai-ujs]

centos 7.6.1810 3.10.0-957.el7.x86_64

copy snapshot of /dev/vdc，kernel panic

[nickchen-cpu]

please try xfs_repair it and then try again.

[lishuai-ujs]

I add flow print code in cow_read_mapping and find the pos is out of bound of array when kernel panic.

[lishuai-ujs]

accord to the vmcore, cow_read_mapping is called from snap_read_bio_get_mode

I add flow trace code and find the curr_end_bytes is uncorrect in second cycle of while loop, curr_end_bytes shound minus the bytes have read in first cycle.

[dakotarwilliams]

Resolved #287