net/ipv4/igmp.c: Replace rcu_dereference() with rcu_access_pointer()
The "rcu_dereference()" call is used directly in a condition.
Since its return value is never dereferenced it is recommended to use
"rcu_access_pointer()" instead of "rcu_dereference()".
Therefore, this patch makes the replacement.
The following Coccinelle semantic patch was used:
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