jffs2: use kmemdup

Convert a sequence of kmalloc and memcpy to use kmemdup.

The semantic patch that performs this transformation is:
(http://coccinelle.lip6.fr/)

// <smpl>
@@
expression a,flag,len;
expression arg,e1,e2;
statement S;
@@

  a =
-  \(kmalloc\|kzalloc\)(len,flag)
+  kmemdup(arg,len,flag)
  <... when != a
  if (a == NULL || ...) S
  ...>
- memcpy(a,arg,len+1);
// </smpl>

Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>