When sending an IPI, if a previous IPI is still pending delivery,
native_lapic_ipi_vectored() waits for the previous IPI to be sent.
We've seen a few inexplicable panics with the current timeout of 50 ms.
Increase the timeout to 1 second and make it tunable.
No hardware specification mentions a timeout in this case; I checked
the Intel SDM, Intel MP spec, and Intel x2APIC spec. Linux and illumos
wait forever. In Linux, see __default_send_IPI_shortcut() in
arch/x86/kernel/apic/ipi.c. In illumos, see apic_send_ipi() in
usr/src/uts/i86pc/io/pcplusmp/apic_common.c. However, misbehaving hardware
could hang the system if we wait forever.
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