ocfs2: Use zero-sized array and struct_size() in kzalloc()

ocfs2: Use zero-sized array and struct_size() in kzalloc()

Update the code to use a zero-sized array instead of a pointer in
structure ocfs2_slot_info and use struct_size() in kzalloc().

Notice that one of the more common cases of allocation size calculations
is finding the size of a structure that has a zero-sized array at the
end, along with memory for some number of elements for that array.  For
example:

  struct foo {
      int stuff;
      void *entry[];
  };

  instance = kzalloc(sizeof(struct foo) + sizeof(void *) * count, GFP_KERNEL);

Instead of leaving these open-coded and prone to type mistakes, we can
now use the new struct_size() helper:

  instance = kzalloc(struct_size(instance, entry, count), GFP_KERNEL);

This code was detected with the help of Coccinelle.

Link: http://lkml.kernel.org/r/20190108191903.GA22056@embeddedor
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Mark Fasheh <mfasheh@versity.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Junxiao Bi <junxiao.bi@oracle.com>
Cc: Joseph Qi <joseph.qi@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>