mm/memtest: add results of early memtest to /proc/meminfo

Currently the memtest results were only presented in dmesg.

When running a large fleet of devices without ECC RAM it's currently not
easy to do bulk monitoring for memory corruption.  You have to parse
dmesg, but that's a ring buffer so the error might disappear after some
time.  In general I do not consider dmesg to be a great API to query RAM
status.

In several companies I've seen such errors remain undetected and cause
issues for way too long.  So I think it makes sense to provide a
monitoring API, so that we can safely detect and act upon them.

This adds /proc/meminfo entry which can be easily used by scripts.

Link: https://lkml.kernel.org/r/20230321103430.7130-1-tomas.mudrunka@gmail.com
Signed-off-by: Tomas Mudrunka <tomas.mudrunka@gmail.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/filesystems/proc.rst

@@ -996,6 +996,7 @@ Example output. You may not have all of these fields.
     VmallocUsed:       40444 kB
     VmallocChunk:          0 kB
     Percpu:            29312 kB
+    EarlyMemtestBad:       0 kB
     HardwareCorrupted:     0 kB
     AnonHugePages:   4149248 kB
     ShmemHugePages:        0 kB
@@ -1146,6 +1147,13 @@ VmallocChunk
 Percpu
               Memory allocated to the percpu allocator used to back percpu
               allocations. This stat excludes the cost of metadata.
+EarlyMemtestBad
+              The amount of RAM/memory in kB, that was identified as corrupted
+              by early memtest. If memtest was not run, this field will not
+              be displayed at all. Size is never rounded down to 0 kB.
+              That means if 0 kB is reported, you can safely assume
+              there was at least one pass of memtest and none of the passes
+              found a single faulty byte of RAM.
 HardwareCorrupted
               The amount of RAM/memory in KB, the kernel identifies as
               corrupted.