kho: make sure kho_scratch argument is fully consumed

When specifying fixed sized scratch areas, the parser only parses the
three scratch sizes and ignores the rest of the argument.  This means the
argument can have any bogus trailing characters.

For example, "kho_scratch=256M,512M,512Mfoobar" results in successful
parsing:

    [    0.000000] KHO: scratch areas: lowmem: 256MiB global: 512MiB pernode: 512MiB

It is generally a good idea to parse arguments as strictly as possible. 
In addition, if bogus trailing characters are allowed in the kho_scratch
argument, it is possible that some people might end up using them and
later extensions to the argument format will cause unexpected breakages.

Make sure the argument is fully consumed after all three scratch sizes are
parsed.  With this change, the bogus argument
"kho_scratch=256M,512M,512Mfoobar" results in:

    [    0.000000] Malformed early option 'kho_scratch'

Link: https://lkml.kernel.org/r/20250826123817.64681-1-pratyush@kernel.org
Signed-off-by: Pratyush Yadav <pratyush@kernel.org>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Alexander Graf <graf@amazon.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Changyuan Lyu <changyuanl@google.com>
Cc: Pratyush Yadav <pratyush@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

kernel/kexec_handover.c

@@ -451,6 +451,10 @@ static int __init kho_parse_scratch_size(char *p)
 	if (!total_size)
 		return -EINVAL;
 
+	/* The string should be fully consumed by now. */
+	if (*p)
+		return -EINVAL;
+
 	scratch_size_lowmem = sizes[0];
 	scratch_size_global = sizes[1];
 	scratch_size_pernode = sizes[2];