uml: boot broken due to buffer overrun

mconsole_init() passed 256 bytes as length in os_create_unix_socket, while
the sizeof UNIX_PATH_MAX is 108. This patch fixes that problem and avoids
a big overrun bug reported on UML bootup.

sockaddr_un.sun_path is UNIX_PATH_MAX long which causes the problem.
Reported-by: Vikas K Managutte <[e-mail]>
Reported-by: Sarvesh Kumar Lal Das <[e-mail]>
Signed-off-by: Balbir Singh <[e-mail]>
Reviewed-by: Pekka Enberg <[e-mail]>
Reviewed-by: WANG Cong <[e-mail]>
Cc: Jeff Dike <[e-mail]>
Cc: <[e-mail]> [please check with Jeff]
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

arch/um/drivers/mconsole_kern.c

···
 #include <linux/slab.h>
 #include <linux/syscalls.h>
 #include <linux/utsname.h>
+#include <linux/socket.h>
+#include <linux/un.h>
 #include <linux/workqueue.h>
 #include <linux/mutex.h>
 #include <asm/uaccess.h>
···
 	/* long to avoid size mismatch warnings from gcc */
 	long sock;
 	int err;
-	char file[256];
+	char file[UNIX_PATH_MAX];
 
 	if (umid_file_name("mconsole", file, sizeof(file)))
 		return -1;