coccinelle: api: add device_attr_show script

According to the documentation[1] show() methods of device attributes
should return the number of bytes printed into the buffer. This is
the return value of scnprintf(). show() must not use snprintf()
when formatting the value to be returned to user space. snprintf()
returns the length the resulting string would be, assuming it all
fit into the destination array[2]. scnprintf() return the length of
the string actually created in buf. If one can guarantee that an
overflow will never happen sprintf() can be used otherwise scnprintf().

[1] Documentation/filesystems/sysfs.txt
[2] "snprintf() confusion" https://lwn.net/Articles/69419/



Signed-off-by: Denis Efremov <efremov@linux.com>
Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>