[JFFS2][XATTR] Fix xd->refcnt race condition

[JFFS2][XATTR] Fix xd->refcnt race condition

When xd->refcnt is checked whether this xdatum should be released
or not, atomic_dec_and_lock() is used to ensure holding the
c->erase_completion_lock.

This fix change a specification of delete_xattr_datum().
Previously, it's only called when xd->refcnt equals zero.
(calling it with positive xd->refcnt cause a BUG())
If you applied this patch, the function checks whether
xd->refcnt is zero or not under the spinlock if necessary.
Then, it marks xd DEAD flahs and links with xattr_dead_list
or releases it immediately when xd->refcnt become zero.

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