ipv6/route: Add a missing check on proc_dointvec

While flushing the cache via  ipv6_sysctl_rtcache_flush(), the call
to proc_dointvec() may fail. The fix adds a check that returns the
error, on failure.

Signed-off-by: Aditya Pakki <pakki001@umn.edu>
Signed-off-by: David S. Miller <davem@davemloft.net>