Add scripts/coccinelle/alloc/drop_kmalloc_cast.cocci

The purpose of this semantic patch is to remove
useless casts, as mentioned in the Linux documentation.
See Chapter 14 in Documentation/CodingStyle for more information.

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