printk: Use console_flush_one_record for legacy printer kthread

printk: Use console_flush_one_record for legacy printer kthread

The legacy printer kthread uses console_lock and
__console_flush_and_unlock to flush records to the console. This
approach results in the console_lock being held for the entire
duration of a flush. This can result in large waiting times for
those waiting for console_lock especially where there is a large
volume of records or where the console is slow (e.g. serial). This
contention is observed during boot, as the call to filp_open in
console_on_rootfs will delay progression to userspace until any
in-flight flush is completed.

Let's instead use console_flush_one_record and release/reacquire
the console_lock between records.

On a PocketBeagle 2, with the following boot args:
"console=ttyS2,9600 initcall_debug=1 loglevel=10"

Without this patch:

[    5.613166] +console_on_rootfs/filp_open
[    5.643473] mmc1: SDHCI controller on fa00000.mmc [fa00000.mmc] using ADMA 64-bit
[    5.643823] probe of fa00000.mmc returned 0 after 258244 usecs
[    5.710520] mmc1: new UHS-I speed SDR104 SDHC card at address 5048
[    5.721976] mmcblk1: mmc1:5048 SD32G 29.7 GiB
[    5.747258]  mmcblk1: p1 p2
[    5.753324] probe of mmc1:5048 returned 0 after 40002 usecs
[   15.595240] ti_sci_pm_domains 44043000.system-controller:power-controller: sync_state() pending due to 30040000.pruss
[   15.595282] ti_sci_pm_domains 44043000.system-controller:power-controller: sync_state() pending due to e010000.watchdog
[   15.595297] ti_sci_pm_domains 44043000.system-controller:power-controller: sync_state() pending due to e000000.watchdog
[   15.595437] ti_sci_pm_domains 44043000.system-controller:power-controller: sync_state() pending due to 30300000.crc
[  146.275961] -console_on_rootfs/filp_open ...

and with:

[    5.477122] +console_on_rootfs/filp_open
[    5.595814] mmc1: SDHCI controller on fa00000.mmc [fa00000.mmc] using ADMA 64-bit
[    5.596181] probe of fa00000.mmc returned 0 after 312757 usecs
[    5.662813] mmc1: new UHS-I speed SDR104 SDHC card at address 5048
[    5.674367] mmcblk1: mmc1:5048 SD32G 29.7 GiB
[    5.699320]  mmcblk1: p1 p2
[    5.705494] probe of mmc1:5048 returned 0 after 39987 usecs
[    6.418682] -console_on_rootfs/filp_open ...
...
[   15.593509] ti_sci_pm_domains 44043000.system-controller:power-controller: sync_state() pending due to 30040000.pruss
[   15.593551] ti_sci_pm_domains 44043000.system-controller:power-controller: sync_state() pending due to e010000.watchdog
[   15.593566] ti_sci_pm_domains 44043000.system-controller:power-controller: sync_state() pending due to e000000.watchdog
[   15.593704] ti_sci_pm_domains 44043000.system-controller:power-controller: sync_state() pending due to 30300000.crc

Where I've added a printk surrounding the call in console_on_rootfs
to filp_open.

Suggested-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Andrew Murray <amurray@thegoodpenguin.co.uk>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Reviewed-by: John Ogness <john.ogness@linutronix.de>
Link: https://patch.msgid.link/20251020-printk_legacy_thread_console_lock-v3-3-00f1f0ac055a@thegoodpenguin.co.uk
[pmladek@suse.com: Fixed ordering of variable definition suggested by John.]
Signed-off-by: Petr Mladek <pmladek@suse.com>