ixgbe: check mbuf refcnt when clearing a ring

ixgbe: check mbuf refcnt when clearing a ring

The function to clear the TX ring when a port was being closed, e.g. on
exit in testpmd, was not checking the mbuf refcnt before freeing it.
Since the function in the vector driver to clear the ring after TX does
not set the pointer to NULL post-free, this caused crashes if mbuf
debugging was turned on.

To reproduce the issue, ensure the follow config variables are set:
RTE_IXGBE_INC_VECTOR
RTE_LIBRTE_MBUF_DEBUG
Then compile up and run testpmd using 10G ports with the vector driver.
Start traffic and let some flow through, then type "stop" and "quit" at
the testpmd prompt, and crash will occur. Output below:

	testpmd> quit
	Stopping port 0...done
	Stopping port 1...PANIC in rte_mbuf_sanity_check():
	bad ref cnt
	[New Thread 0x7fffabfff700 (LWP 145312)]
	[New Thread 0x7fffb47fe700 (LWP 145311)]
	[New Thread 0x7fffb4fff700 (LWP 145310)]
	[New Thread 0x7ffff6cd5700 (LWP 145309)]
	18: [/home/bruce/dpdk.org/x86_64-native-linuxapp-gcc/app/testpmd(_start+0x29)
	<....snip for brevity...>
	Program received signal SIGABRT, Aborted.
	0x00007ffff7120a98 in raise () from /lib64/libc.so.6

A similar error occurs when clearing the RX ring, which is also fixed by
this patch.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>