Bump MAXMEMDOM value to 8 to match amd64

[gonzo]

Summary

On some of the server-grade ARM64 machines the number of NUMA domains
is higher than 2. When booting GENERIC kernel on such machines the SRAT
parser fails leaving the system with a single domain. To make GENERIC kernel
usable on those server, match the parameter value with the one for amd64 arch.

Test Plan

Tested on Ampere Altra

[alc]

I'd be interested in seeing the output of sysctl vm.phys_segs on one of these machines. Can you post it here?

[gonzo]

vm.phys_segs:
SEGMENT 0:

start:     0x90000000
end:       0x92000000
domain:    0
free list: 0xffff000000d06400

SEGMENT 1:

start:     0x92780000
end:       0xab600000
domain:    0
free list: 0xffff000000d06400

SEGMENT 2:

start:     0xacf42000
end:       0xba1a0000
domain:    0
free list: 0xffff000000d06400

SEGMENT 3:

start:     0xbf770000
end:       0xbf8a0000
domain:    0
free list: 0xffff000000d06400

SEGMENT 4:

start:     0xbfd30000
end:       0xc0000000
domain:    0
free list: 0xffff000000d06400

SEGMENT 5:

start:     0x80000000000
end:       0x80080000000
domain:    0
free list: 0xffff000000d06400

SEGMENT 6:

start:     0x800c0000000
end:       0x81000000000
domain:    0
free list: 0xffff000000d06400

SEGMENT 7:

start:     0xa0000000000
end:       0xa0e6926f000
domain:    1
free list: 0xffff000000d06640

SEGMENT 8:

start:     0xc0000000000
end:       0xc1000000000
domain:    2
free list: 0xffff000000d06880

SEGMENT 9:

start:     0xe0000000000
end:       0xe0fffffe000
domain:    3
free list: 0xffff000000d06ac0

[allanjude]

In D27368#613064, @alc wrote:

I'd be interested in seeing the output of sysctl vm.phys_segs on one of these machines. Can you post it here?

https://reviews.freebsd.org/P463