drivers/block/zram/zram_drv.c: fix bug storing backing_dev

The call to strlcpy in backing_dev_store is incorrect. It should take
the size of the destination buffer instead of the size of the source
buffer.  Additionally, ignore the newline character (\n) when reading
the new file_name buffer. This makes it possible to set the backing_dev
as follows:

	echo /dev/sdX > /sys/block/zram0/backing_dev

The reason it worked before was the fact that strlcpy() copies 'len - 1'
bytes, which is strlen(buf) - 1 in our case, so it accidentally didn't
copy the trailing new line symbol.  Which also means that "echo -n
/dev/sdX" most likely was broken.

Signed-off-by: Peter Kalauskas <peskal@google.com>
Link: http://lkml.kernel.org/r/20180813061623.GC64836@rodete-desktop-imager.corp.google.com
Acked-by: Minchan Kim <minchan@kernel.org>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: <stable@vger.kernel.org>    [4.14+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/block/zram/zram_drv.c

@@ -337,6 +337,7 @@ static ssize_t backing_dev_store(struct device *dev,
 		struct device_attribute *attr, const char *buf, size_t len)
 {
 	char *file_name;
+	size_t sz;
 	struct file *backing_dev = NULL;
 	struct inode *inode;
 	struct address_space *mapping;
@@ -357,7 +358,11 @@ static ssize_t backing_dev_store(struct device *dev,
 		goto out;
 	}
 
-	strlcpy(file_name, buf, len);
+	strlcpy(file_name, buf, PATH_MAX);
+	/* ignore trailing newline */
+	sz = strlen(file_name);
+	if (sz > 0 && file_name[sz - 1] == '\n')
+		file_name[sz - 1] = 0x00;
 
 	backing_dev = filp_open(file_name, O_RDWR|O_LARGEFILE, 0);
 	if (IS_ERR(backing_dev)) {