netmem: use _Generic to cover const casting for page_to_netmem()

The current page_to_netmem() doesn't cover const casting resulting in
trying to cast const struct page * to const netmem_ref fails.

To cover the case, change page_to_netmem() to use macro and _Generic.

Signed-off-by: Byungchul Park <[e-mail]>
Reviewed-by: Mina Almasry <[e-mail]>
Reviewed-by: Toke Høiland-Jørgensen <[e-mail]>
Reviewed-by: Pavel Begunkov <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

include/net/netmem.h

···
 	return (__force netmem_ref)((unsigned long)niov | NET_IOV);
 }
 
-static inline netmem_ref page_to_netmem(const struct page *page)
-{
-	return (__force netmem_ref)page;
-}
+#define page_to_netmem(p)	(_Generic((p),			\
+	const struct page * :	(__force const netmem_ref)(p),	\
+	struct page * :		(__force netmem_ref)(p)))
 
 /**
  * virt_to_netmem - convert virtual memory pointer to a netmem reference