tty: serial: Use GFP_ATOMIC instead of GFP_KERNEL in serial8250_em485_init()

serial8250_em485_init() is supposed to be protected with
p->port.lock spinlock.
This may lead to issues when kmalloc sleeps, so it is better to use
GFP_ATOMIC in this spinlocked context.

Fixes: e490c914 ("tty: Add software emulated RS485 support for 8250")
Reported-by: Ильяс Гасанов <torso.nafi@gmail.com>
Signed-off-by: Matwey V. Kornilov <matwey@sai.msu.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_port.c

@@ -588,7 +588,7 @@ int serial8250_em485_init(struct uart_8250_port *p)
 	if (p->em485 != NULL)
 		return 0;
 
-	p->em485 = kmalloc(sizeof(struct uart_8250_em485), GFP_KERNEL);
+	p->em485 = kmalloc(sizeof(struct uart_8250_em485), GFP_ATOMIC);
 	if (p->em485 == NULL)
 		return -ENOMEM;