kdump: return -ENOENT if required cmdline option does not exist

According to the current crashkernel=Y,low support in other ARCHes, it's
an optional command-line option. When it doesn't exist, kernel will try
to allocate minimum required memory below 4G automatically.

However, __parse_crashkernel() returns '-EINVAL' for all error cases. It
can't distinguish the nonexistent option from invalid option.

Change __parse_crashkernel() to return '-ENOENT' for the nonexistent option
case. With this change, crashkernel,low memory will take the default
value if crashkernel=,low is not specified; while crashkernel reservation
will fail and bail out if an invalid option is specified.
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Acked-by: Baoquan He <bhe@redhat.com>
Link: https://lore.kernel.org/r/20220506114402.365-2-thunder.leizhen@huawei.comSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

kernel/crash_core.c

@@ -243,9 +243,8 @@ static int __init __parse_crashkernel(char *cmdline,
 	*crash_base = 0;
 
 	ck_cmdline = get_last_crashkernel(cmdline, name, suffix);
-
 	if (!ck_cmdline)
-		return -EINVAL;
+		return -ENOENT;
 
 	ck_cmdline += strlen(name);