CLEANUP: fdtab: flatten the struct and merge the spec struct with the rest

CLEANUP: fdtab: flatten the struct and merge the spec struct with the rest

The "spec" sub-struct was using 8 bytes for only 5 needed. There is no
reason to keep it as a struct, it doesn't bring any value. By flattening
it, we can merge the single byte with the next single byte, resulting in
an immediate saving of 4 bytes (20%). Interestingly, tests have shown a
steady performance gain of 0.6% after this change, which can possibly be
attributed to a more cache-line friendly struct.