netlink: fix stack buffer overflow with sub-reg sized prefixes

netlink: fix stack buffer overflow with sub-reg sized prefixes

The calculation of the dynamic on-stack array is incorrect,
the scratch space can be too low which gives stack corruption:

AddressSanitizer: dynamic-stack-buffer-overflow on address 0x7ffdb454f064..
    #1 0x7fabe92aaac4 in __mpz_export_data src/gmputil.c:108
    #2 0x7fabe92d71b1 in netlink_export_pad src/netlink.c:251
    #3 0x7fabe92d91d8 in netlink_gen_prefix src/netlink.c:476

div_round_up() cannot be used here, it fails to account for register
padding.  A 16 bit prefix will need 2 registers (start, end -- 8 bytes
in total).

Remove the dynamic sizing and add an assertion in case upperlayer
ever passes invalid expr sizes down to us.

After this fix, the combination is rejected by the kernel
because of the maps' wrong data size, before the fix userspace
may crash before.

Signed-off-by: Florian Westphal <fw@strlen.de>