ipv4: annotate a few functions __init in ipconfig.c

A few functions are only used from __init context.
So annotate these with __init for consistency and silence
the following warnings:

WARNING: net/ipv4/built-in.o(.text+0x2a876): Section mismatch
         in reference from the function ic_bootp_init() to
         the variable .init.data:bootp_packet_type
WARNING: net/ipv4/built-in.o(.text+0x2a907): Section mismatch
         in reference from the function ic_bootp_cleanup() to
         the variable .init.data:bootp_packet_type

Note: The warnings only appear with CONFIG_DEBUG_SECTION_MISMATCH=y

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>