Coccinelle: Add iterators/list_entry_update.cocci

Coccinelle: Add iterators/list_entry_update.cocci

list_for_each_entry uses its first argument to get from one element of
the list to the next, so it is usually not a good idea to reassign it.
The first rule finds such a reassignment and the second rule checks
that there is a path from the reassignment back to the top of the loop.

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