x86: irq: Get correct available vectors for cpu disable

check_irq_vectors_for_cpu_disable() can overestimate the number of
available interrupt vectors, so the check for cpu down succeeds, but
the actual cpu removal fails.

It iterates from FIRST_EXTERNAL_VECTOR to NR_VECTORS, which is wrong
because the systems vectors are not taken into account.

Limit the search to first_system_vector instead of NR_VECTORS.

The second indicator for vector availability the used_vectors bitmap
is not taken into account at all. So system vectors,
e.g. IA32_SYSCALL_VECTOR (0x80) and IRQ_MOVE_CLEANUP_VECTOR (0x20),
are accounted as available.

Add a check for the used_vectors bitmap and do not account vectors
which are marked there.

[ tglx: Simplified code. Rewrote changelog and code comments. ]

Signed-off-by: Yinghai Lu <[email protected]>
Acked-by: Prarit Bhargava <[email protected]>
Cc: Seiji Aguchi <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: K. Y. Srinivasan <[email protected]>
Cc: Steven Rostedt (Red Hat) <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: "Elliott, Robert (Server Storage)" <[email protected]>
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Thomas Gleixner <[email protected]>

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