[FS#4158] Master regression - boot loop due to kernel panic (mt7621)

[OpenWrt Bugs]

THIS IS AN AUTOMATED MESSAGE, DO NOT REPLY.

The following task has a new comment added:

FS#4158 - Master regression - boot loop due to kernel panic (mt7621)
User who did this - d.souza (dsouza)

----------
I just did a test, and as expected disabling HighMem support in **./target/linux/ramips/mt7621/config-5.4 **is a temporary workaround:

**CONFIG_HIGHMEM=n**

This was enabled in the previous builds and it was working. I believe there is some change in the kernel or some patch that is causing HighMem detection to fail. I've tracked this to the following code, but the code below was not changed and I have no understanding about the logic of the code below to proceed with the investigation to understand what caused it started failing (since it hasn't changed):

**/arch/mips/ralink/mt7621.c**

static void __init mt7621_memory_detect(void)
{
	void *dm = &detect_magic;
	phys_addr_t size;

	for (size = 32 * SZ_1M; size < 256 * SZ_1M; size