efi: Don't map the entire mokvar table to determine its size

In the Linux kernel, the following vulnerability has been resolved:

efi: Don't map the entire mokvar table to determine its size

Currently, when validating the mokvar table, we (re)map the entire table

on each iteration of the loop, adding space as we discover new entries.

If the table grows over a certain size, this fails due to limitations of

early_memmap(), and we get a failure and traceback:

------------[ cut here ]------------

WARNING: CPU: 0 PID: 0 at mm/early_ioremap.c:139 __early_ioremap+0xef/0x220

...

Call Trace:

<TASK>

? __early_ioremap+0xef/0x220

? __warn.cold+0x93/0xfa

? __early_ioremap+0xef/0x220

? report_bug+0xff/0x140

? early_fixup_exception+0x5d/0xb0

? early_idt_handler_common+0x2f/0x3a

? __early_ioremap+0xef/0x220

? efi_mokvar_table_init+0xce/0x1d0

? setup_arch+0x864/0xc10

? start_kernel+0x6b/0xa10

? x86_64_start_reservations+0x24/0x30

? x86_64_start_kernel+0xed/0xf0

? common_startup_64+0x13e/0x141

</TASK>

---[ end trace 0000000000000000 ]---

mokvar: Failed to map EFI MOKvar config table pa=0x7c4c3000, size=265187.

Mapping the entire structure isn't actually necessary, as we don't ever

need more than one entry header mapped at once.

Changes efi_mokvar_table_init() to only map each entry header, not the

entire table, when determining the table size. Since we're not mapping

any data past the variable name, it also changes the code to enforce

that each variable name is NUL terminated, rather than attempting to

verify it in place.