pstore/ram: Check start of empty przs during init

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

pstore/ram: Check start of empty przs during init

After commit 30696378f68a ("pstore/ram: Do not treat empty buffers as

valid"), initialization would assume a prz was valid after seeing that

the buffer_size is zero (regardless of the buffer start position). This

unchecked start value means it could be outside the bounds of the buffer,

leading to future access panics when written to:

sysdump_panic_event+0x3b4/0x5b8

atomic_notifier_call_chain+0x54/0x90

panic+0x1c8/0x42c

die+0x29c/0x2a8

die_kernel_fault+0x68/0x78

__do_kernel_fault+0x1c4/0x1e0

do_bad_area+0x40/0x100

do_translation_fault+0x68/0x80

do_mem_abort+0x68/0xf8

el1_da+0x1c/0xc0

__raw_writeb+0x38/0x174

__memcpy_toio+0x40/0xac

persistent_ram_update+0x44/0x12c

persistent_ram_write+0x1a8/0x1b8

ramoops_pstore_write+0x198/0x1e8

pstore_console_write+0x94/0xe0

...

To avoid this, also check if the prz start is 0 during the initialization

phase. If not, the next prz sanity check case will discover it (start >

size) and zap the buffer back to a sane state.

[kees: update commit log with backtrace and clarifications]