fs/select.c: use struct_size() in kmalloc()

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;
       struct boo entry[];
  };

  size = sizeof(struct foo) + count * sizeof(struct boo);
  instance = kmalloc(size, GFP_KERNEL);

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

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

Also, notice that variable size is unnecessary, hence it is removed.

This code was detected with the help of Coccinelle.

Link: http://lkml.kernel.org/r/20190604164226.GA13823@embeddedorSigned-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/select.c

@@ -961,7 +961,7 @@ static int do_sys_poll(struct pollfd __user *ufds, unsigned int nfds,
 		struct timespec64 *end_time)
 {
 	struct poll_wqueues table;
- 	int err = -EFAULT, fdcount, len, size;
+	int err = -EFAULT, fdcount, len;
 	/* Allocate small arguments on the stack to save memory and be
 	   faster - use long to make sure the buffer is aligned properly
 	   on 64 bit archs to avoid unaligned access */
@@ -989,8 +989,8 @@ static int do_sys_poll(struct pollfd __user *ufds, unsigned int nfds,
 			break;
 
 		len = min(todo, POLLFD_PER_PAGE);
-		size = sizeof(struct poll_list) + sizeof(struct pollfd) * len;
-		walk = walk->next = kmalloc(size, GFP_KERNEL);
+		walk = walk->next = kmalloc(struct_size(walk, entries, len),
+					    GFP_KERNEL);
 		if (!walk) {
 			err = -ENOMEM;
 			goto out_fds;