[JFFS2][XATTR] Fix memory leak in POSIX-ACL support

jffs2_clear_acl() which releases acl caches allocated by kmalloc()
was defined but it was never called. Thus, we faced to the risk
of memory leaking.

This patch plugs jffs2_clear_acl() into jffs2_do_clear_inode().
It ensures to release acl cache when inode is cleared.

Signed-off-by: KaiGai Kohei <kaigai@ak.jp.nec.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>