Coccinelle: Add free/kfree.cocci

Find a use after free.  Values of variables may imply that some
execution paths are not possible, resulting in false positives.
Another source of false positives are macros such as
SCTP_DBG_OBJCNT_DEC that do not actually evaluate their argument

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