fwcontrol: Allocate full fw_asyreq structures passed to the kernel

The FW_ASYREQ ioctl accepts a struct fw_asyreq object as its argument,
meaning that the kernel always copies in the full structure in
sys_ioctl before passing the request down to the driver. However,
fwcontrol was allocating smaller objects that contained only the
request header and a variable-sized payload. This means that the
kernel copy in sys_ioctl was reading off the end of this buffer. On
current architectures this happened to be ok, but it is UB.

Instead, allocate a full structure.

Reported by: GCC 14 -Walloc-size
Reviewed by: rlibby, brooks
Differential Revision: https://reviews.freebsd.org/D46014