fix a panic when passing ifioctl from a netmap file descriptor to
the underlying device. This needs to be merged to 10.1

Reported by: Patrick Kelsey
MFC after:	3 days


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272111 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f