net: fixup address-space warnings in compat_mc_{get,set}sockopt()

Add __user attributes in some of the casts in this function to avoid
the following sparse warnings:

net/compat.c:592:57: warning: cast removes address space of expression
net/compat.c:592:57: warning: incorrect type in initializer (different address spaces)
net/compat.c:592:57:    expected struct compat_group_req [noderef] <asn:1>*gr32
net/compat.c:592:57:    got void *<noident>
net/compat.c:613:65: warning: cast removes address space of expression
net/compat.c:613:65: warning: incorrect type in initializer (different address spaces)
net/compat.c:613:65:    expected struct compat_group_source_req [noderef] <asn:1>*gsr32
net/compat.c:613:65:    got void *<noident>
net/compat.c:634:60: warning: cast removes address space of expression
net/compat.c:634:60: warning: incorrect type in initializer (different address spaces)
net/compat.c:634:60:    expected struct compat_group_filter [noderef] <asn:1>*gf32
net/compat.c:634:60:    got void *<noident>
net/compat.c:672:52: warning: cast removes address space of expression
net/compat.c:672:52: warning: incorrect type in initializer (different address spaces)
net/compat.c:672:52:    expected struct compat_group_filter [noderef] <asn:1>*gf32
net/compat.c:672:52:    got void *<noident>

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>