Add scripts/coccinelle/err_cast.cocci

Add a Coccinelle file to use the ERR_CAST function

Before the release 2.6.25, one had to use ERR_PTR(PTR_ERR(...))  to
convert the pointer type of an error. Since then, the function
ERR_CAST has been available for that purpose.

Signed-off-by: Nicolas Palix <npalix@diku.dk>
Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: Michal Marek <mmarek@suse.cz>