ipv6: Remove unused var in inet6_dump_ifinfo()

Reported by Stephen Rothwell:

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Today's linux-next build (x86_64 allmodconfig) produced this warning:

net/ipv6/addrconf.c: In function 'inet6_dump_ifinfo':
net/ipv6/addrconf.c:3833: warning: unused variable 'err'

Introduced by commit 84d2697d ("ipv6:
speedup inet6_dump_ifinfo()").
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Signed-off-by: David S. Miller <davem@davemloft.net>