The devstat API expects us to reuse one devinfo for many invocations of
devstat_getstats. In particular, it allocates and resizes memory
referenced by devinfo.
Querying the number of devices separately from the device list itself is
racy. Devices may be added or removed between the two calls; and removed
devices would lead to a segfault.
The memory allocated by calloc was never freed. Since the devinfo struct
never leaves the function, anyway, we might as well just allocate it on
the stack.
