cxgbe's native netmap support broken since r307394

[Navdeep Parhar]

Luigi, Vincenzo,

The last major update to netmap (r307394 and followups) broke cxgbe's
native netmap support.  The problem is that netmap_hw_reg now holds an
rw_lock around the driver's netmap_on/off routines.  It has always been
safe for the driver to sleep during these operations but now it panics
instead.

Why is IFNET_WLOCK needed here?  It seems like a regression to disallow
sleep on the control path.

Regards,
Navdeep

begin_synchronized_op with the following non-sleepable locks held:
exclusive rw ifnet_rw (ifnet_rw) r = 0 (0xffffffff8271d680) locked @
/root/ws/head/sys/dev/netmap/netmap_freebsd.c:95
stack backtrace:
#0 0xffffffff810837a5 at witness_debugger+0xe5
#1 0xffffffff81084d88 at witness_warn+0x3b8
#2 0xffffffff83ef2bcc at begin_synchronized_op+0x6c
#3 0xffffffff83f14beb at cxgbe_netmap_reg+0x5b
#4 0xffffffff809846f1 at netmap_hw_reg+0x81
#5 0xffffffff809806de at netmap_do_regif+0x19e
#6 0xffffffff8098121d at netmap_ioctl+0x7ad
#7 0xffffffff8098682f at freebsd_netmap_ioctl+0x5f