etc/fstab/GRMLBASE: use reserved IP following RFC5737

Usage of IP addresses like 1.2.3.4 is not recommended, as they
might point to actually used and non-reserved IPs.

Quoting from RFC 5737 AKA "IPv4 Address Blocks Reserved for
Documentation" (see https://datatracker.ietf.org/doc/html/rfc5737):

| The blocks 192.0.2.0/24 (TEST-NET-1), 198.51.100.0/24 (TEST-NET-2), and
| 203.0.113.0/24 (TEST-NET-3) are provided for use in documentation.

So use 192.0.2.42 instead.