tty: n_gsm: Debug output allocation must use GFP_ATOMIC

Dan Carpenter <dan.carpenter@oracle.com> reported the following Smatch
warning:

drivers/tty/n_gsm.c:720 gsm_data_kick()
warn: sleeping in atomic context

This is because gsm_control_message() is holding a spin lock so
gsm_hex_dump_bytes() needs to use GFP_ATOMIC instead of GFP_KERNEL.

Fixes: 925ea0f ("tty: n_gsm: Fix packet data hex dump output")
Cc: stable <stable@kernel.org>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Link: https://lore.kernel.org/r/20220523155052.57129-1-tony@atomide.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: tmlind

drivers/tty/n_gsm.c

@@ -455,7 +455,7 @@ static void gsm_hex_dump_bytes(const char *fname, const u8 *data,
 		return;
 	}
 
-	prefix = kasprintf(GFP_KERNEL, "%s: ", fname);
+	prefix = kasprintf(GFP_ATOMIC, "%s: ", fname);
 	if (!prefix)
 		return;
 	print_hex_dump(KERN_INFO, prefix, DUMP_PREFIX_OFFSET, 16, 1, data, len,