net: core: dev.c: fix a documentation warning

There's a markup for link with is "foo_". On this kernel-doc
comment, we don't want this, but instead, place a literal
reference. So, escape the literal with ``foo``, in order to
avoid this warning:

	./net/core/dev.c:5195: WARNING: Unknown target name: "page_is".
Signed-off-by: NMauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -5195,7 +5195,7 @@ static int __netif_receive_skb_one_core(struct sk_buff *skb, bool pfmemalloc)
  *
  *	More direct receive version of netif_receive_skb().  It should
  *	only be used by callers that have a need to skip RPS and Generic XDP.
- *	Caller must also take care of handling if (page_is_)pfmemalloc.
+ *	Caller must also take care of handling if ``(page_is_)pfmemalloc``.
  *
  *	This function may only be called from softirq context and interrupts
  *	should be enabled.