printk: Save interrupt flags when enabling __log_oops_buf

oops_printk_start() may be called with interrupts disabled; save and
restore the interrupt flags properly.

Found after a spinlock was acquired recursively in the following backtrace:

\raw_spin_lock_irq
\run_timer_softirq
...(skipped)
\el1_irq
()
\printk\oops_printk_start
\panic\oops_enter
\traps\die
\fault\__do_kernel_fault.part.5
\fault\do_page_fault
\fault\do_translation_fault
\fault\do_mem_abort
()
\get_next_timer_interrupt
\tick-sched\__tick_nohz_idle_enter
\tick-sched\tick_nohz_idle_enter
\idle\cpu_startup_entry
\init/main\rest_init
\init/main\start_kernel

CRs-fixed: 754837
Change-Id: Ib9b5079b0177833d40b14ddf0f0458be5676509a
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>