ovl: Use "buf" flexible array for memcpy() destination

The "buf" flexible array needs to be the memcpy() destination to avoid
false positive run-time warning from the recent FORTIFY_SOURCE
hardening:

  memcpy: detected field-spanning write (size 93) of single field "&fh->fb"
  at fs/overlayfs/export.c:799 (size 21)

Reported-by:  <syzbot+9d14351a171d0d1c7955@syzkaller.appspotmail.com>
Link: https://lore.kernel.org/all/000000000000763a6c05e95a5985@google.com/


Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>