an efi_devdesc structure. When we're netbooting, f->f_devdata holds
the address of the network socket variable. Dereferencing this caused
some very unpredictable behaviour, including proper functioning.
So, as a sanity check, we first make sure f->f_dev points to our
own devsw. If not, the open will fail before we use f->f_devdata.
This solves the netboot hangs I invariably got whenever I used the
latest toolchain to compile the EFI loader.
