net: make sock_prot_memory_pressure() return "const char *"

This function returns string literals which are "const char *".
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/sock.c

@@ -3497,7 +3497,7 @@ static long sock_prot_memory_allocated(struct proto *proto)
 	return proto->memory_allocated != NULL ? proto_memory_allocated(proto) : -1L;
 }
 
-static char *sock_prot_memory_pressure(struct proto *proto)
+static const char *sock_prot_memory_pressure(struct proto *proto)
 {
 	return proto->memory_pressure != NULL ?
 	proto_memory_pressure(proto) ? "yes" : "no" : "NI";