ipv6: snmp: remove icmp6type2name[]

ipv6: snmp: remove icmp6type2name[]

This 2KB array can be replaced by a switch() to save space.

Before:
$ size net/ipv6/proc.o
   text	   data	    bss	    dec	    hex	filename
   6410	    624	      0	   7034	   1b7a	net/ipv6/proc.o

After:
$ size net/ipv6/proc.o
   text	   data	    bss	    dec	    hex	filename
   5516	    592	      0	   6108	   17dc	net/ipv6/proc.o

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Link: https://patch.msgid.link/20250905165813.1470708-2-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>