ftrace: ipipe: enable tracing from the head domain
Enabling ftrace for a co-kernel running in the head domain of a pipelined interrupt context means to: - make sure that ftrace's live kernel code patching still runs unpreempted by any head domain activity (so that the latter can't tread on invalid or half-baked changes in the .text section). - allow the co-kernel code running in the head domain to traverse ftrace's tracepoints safely. The changes introduced by this commit ensure this by fixing up some key critical sections so that interrupts are still disabled in the CPU, undoing the interrupt flag virtualization in those particular cases.
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