Fix off-by-one in readlink() buffer size handling

Fix off-by-one in readlink() buffer size handling

readlink() returns the number of bytes in the buffer.

If we do something like

len = readlink(path, buf, sizeof(buf));
buf[len] = '\0';

we might write one byte past the end of the buffer.

Signed-off-by: Thomas Jarosch <thomas.jarosch@intra2net.com>
Signed-off-by: NeilBrown <neilb@suse.de>