eloop: Fix fd_table allocation for epoll and kqueue

eloop: Fix fd_table allocation for epoll and kqueue

The previous implementation did not work if the first registered socket
had fd > 16 or if the fd was more than double the largest value used in
previous registrations. Those cases could result in too small a memory
allocation being used and writes/reads beyond the end of that buffer.

This fix is applicable to CONFIG_ELOOP_EPOLL=y and CONFIG_ELOOP_KQUEUE=y
builds.

Fixes: f0356ec85c46 ("eloop: Add epoll option for better performance")
Signed-off-by: Jouni Malinen <j@w1.fi>