tty: n_hdlc: Use flexible-array member

Old code in the kernel uses 1-byte and 0-byte arrays to indicate the
presence of a "variable length array":

struct something {
    int length;
    u8 data[1];
};

struct something *instance;

instance = kmalloc(sizeof(*instance) + size, GFP_KERNEL);
instance->length = size;
memcpy(instance->data, source, size);

There is also 0-byte arrays. Both cases pose confusion for things like
sizeof(), CONFIG_FORTIFY_SOURCE, etc.[1] Instead, the preferred mechanism
to declare variable-length types such as the one above is a flexible array
member[2] which need to be the last member of a structure and empty-sized:

struct something {
        int stuff;
        u8 data[];
};

Also, by making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
unadvertenly introduced[3] to the codebase from now on.

[1] KSPP#21
[2] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[3] commit 7649773 ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <[email protected]>

drivers/tty/n_hdlc.c

@@ -115,7 +115,7 @@
 struct n_hdlc_buf {
 	struct list_head  list_item;
 	int		  count;
-	char		  buf[1];
+	char		  buf[];
 };
 
 #define	N_HDLC_BUF_SIZE	(sizeof(struct n_hdlc_buf) + maxframe)