[MAJOR] spec I/O: fix allocations of spec entries for an FD

[MAJOR] spec I/O: fix allocations of spec entries for an FD

Under some circumstances, it was possible with speculative I/O to
reallocate multiple entries for the same FD if an fd_{set,clr,set}
or fd_{clr,set,clr} sequences were performed before a schedule.

Fix this by keeping a an allocation flag for each fd.