EFI/CPER: don't dump the entire memory region

In the Linux kernel, the following vulnerability has been resolved: EFI/CPER: don't dump the entire memory region The current logic at cper_print_fw_err() doesn't check if the error record length is big enough to handle offset. On a bad firmware, if the ofset is above the actual record, length -= offset will underflow, making it dump the entire memory. The end result can be: - the logic taking a lot of time dumping large regions of memory; - data disclosure due to the memory dumps; - an OOPS, if it tries to dump an unmapped memory region. Fix it by checking if the section length is too small before doing a hex dump. [ rjw: Subject tweaks ]