/proc/sysrq-trigger: accept multiple keys at once

/proc/sysrq-trigger: accept multiple keys at once

This way we can do:
`echo _reisub > /proc/sysrq-trigger`
Instead of:
`for i in r e i s u b; do echo "$i" > /proc/sysrq-trigger; done;`

This can be very useful when trying to execute sysrq combo remotely
or from userspace. When sending keys in multiple separate writes,
userspace (eg. bash or ssh) can be killed before whole combo is completed.
Therefore putting all keys in single write is more robust approach.

Signed-off-by: Tomas Mudrunka <tomas.mudrunka@gmail.com>
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20231120111451.527952-1-tomas.mudrunka@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>