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 ]