net: ipconfig: Don't override command-line hostnames or domains

[ Upstream commit b508d5fb ]

If the user specifies a hostname or domain name as part of the ip=
command-line option, preserve it and don't overwrite it with one
supplied by DHCP/BOOTP.

For instance, ip=::::myhostname::dhcp will use "myhostname" rather than
ignoring and overwriting it.

Fix the comment on ic_bootp_string that suggests it only copies a string
"if not already set"; it doesn't have any such logic.

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Lee Jones <joneslee@google.com>
Change-Id: I5ae33183ed568b13690ea4e4b4ea37c4a28697ba