net-procfs: Use vsnprintf extension %phN

Save a bit of code by using the kernel extension.

$ size net/core/net-procfs.o*
   text	   data	    bss	    dec	    hex	filename
   3701	    120	      0	   3821	    eed	net/core/net-procfs.o.new
   3764	    120	      0	   3884	    f2c	net/core/net-procfs.o.old

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: David S. Miller <davem@davemloft.net>