ubifs: Use kmalloc_array()

Since commit 6da2ec56 ("treewide: kmalloc() -> kmalloc_array()")
we use kmalloc_array() for kmalloc() that computes the length with
a multiplication.

Cc: Kees Cook <keescook@chromium.org>
Suggested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Richard Weinberger <richard@nod.at>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Richard Weinberger <richard@nod.at>