netmap_ioctl: Fix ifnet reference leak on failure.
netmap_ioctl: Fix ifnet reference leak on failure.

The redefinition of ifp variable in the registration path was shadowing
a definition of the same variable at the start of netmap_ioctl function
and in case of netmap_do_regif failure the error path was trying to
unref the outer variable which contains NULL, while the ifnet object was
pointed by the inner ifp variable and as it went out of scope the ifnet
object will have one extra reference.

luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap