x86/console: process softirqs between warning prints

x86/console: process softirqs between warning prints

Process softirqs while printing end of boot warnings. Each warning can
be several lines long, and on slow consoles printing multiple ones
without processing softirqs can result in the watchdog triggering:

(XEN) [   22.277806] ***************************************************
(XEN) [   22.417802] WARNING: CONSOLE OUTPUT IS SYNCHRONOUS
(XEN) [   22.556029] This option is intended to aid debugging of Xen by ensuring
(XEN) [   22.696802] that all output is synchronously delivered on the serial line.
(XEN) [   22.838024] However it can introduce SIGNIFICANT latencies and affect
(XEN) [   22.978710] timekeeping. It is NOT recommended for production use!
(XEN) [   23.119066] ***************************************************
(XEN) [   23.258865] Booted on L1TF-vulnerable hardware with SMT/Hyperthreading
(XEN) [   23.399560] enabled.  Please assess your configuration and choose an
(XEN) [   23.539925] explicit 'smt=<bool>' setting.  See XSA-273.
(XEN) [   23.678860] ***************************************************
(XEN) [   23.818492] Booted on MLPDS/MFBDS-vulnerable hardware with SMT/Hyperthreading
(XEN) [   23.959811] enabled.  Mitigations will not be fully effective.  Please
(XEN) [   24.100396] choose an explicit smt=<bool> setting.  See XSA-297.
(XEN) [   24.240254] *************************************************(XEN) [   24.247302] Watchdog timer detects that CPU0 is stuck!
(XEN) [   24.386785] ----[ Xen-4.17-unstable  x86_64  debug=y  Tainted:   C    ]----
(XEN) [   24.527874] CPU:    0
(XEN) [   24.662422] RIP:    e008:[<ffff82d04025b84a>] drivers/char/ns16550.c#ns16550_tx_ready+0x3a/0x90

Fixes: ee3fd57acd ('xen: add warning infrastructure')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>