fix multiplication overflow in copy_fdtable()

cpy and set really should be size_t; we won't get an overflow on that,
since sysctl_nr_open can't be set above ~(size_t)0 / sizeof(void *),
so nr that would've managed to overflow size_t on that multiplication
won't get anywhere near copy_fdtable() - we'll fail with EMFILE
before that.

Cc: stable@kernel.org # v2.6.25+
Fixes: 9cfe015 (get rid of NR_OPEN and introduce a sysctl_nr_open)
Reported-by: Thiago Macieira <thiago.macieira@intel.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

Author: Al Viro

fs/file.c

@@ -70,7 +70,7 @@ static void copy_fd_bitmaps(struct fdtable *nfdt, struct fdtable *ofdt,
  */
 static void copy_fdtable(struct fdtable *nfdt, struct fdtable *ofdt)
 {
-	unsigned int cpy, set;
+	size_t cpy, set;
 
 	BUG_ON(nfdt->max_fds < ofdt->max_fds);