tty: xilinx_uartps: split sysrq handling

In the Linux kernel, the following vulnerability has been resolved:

tty: xilinx_uartps: split sysrq handling

lockdep detects the following circular locking dependency:

CPU 0 CPU 1

========================== ============================

cdns_uart_isr() printk()

uart_port_lock(port) console_lock()

cdns_uart_console_write()

if (!port->sysrq)

uart_port_lock(port)

uart_handle_break()

port->sysrq = ...

uart_handle_sysrq_char()

printk()

console_lock()

The fixed commit attempts to avoid this situation by only taking the

port lock in cdns_uart_console_write if port->sysrq unset. However, if

(as shown above) cdns_uart_console_write runs before port->sysrq is set,

then it will try to take the port lock anyway. This may result in a

deadlock.

Fix this by splitting sysrq handling into two parts. We use the prepare

helper under the port lock and defer handling until we release the lock.