apparmor: fix memory leak on buffer on error exit path

Currently on the error exit path the allocated buffer is not free'd
causing a memory leak. Fix this by kfree'ing it.

Detected by CoverityScan, CID#1466876 ("Resource leaks")

Fixes: 1180b4c757aa ("apparmor: fix dangling symlinks to policy rawdata after replacement")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>