[JFFS2][XATTR] Remove jffs2_garbage_collect_xattr(c, ic)

Remove jffs2_garbage_collect_xattr(c, ic).
jffs2_garbage_collect_xattr_datum/ref() are called from gc.c directly.

In original implementation, jffs2_garbage_collect_xattr(c, ic) returns
with holding a spinlock if 'ic' is inode_cache. But it returns after
releasing a spinlock if 'ic' is xattr_datum/ref.
It looks so confusable behavior. Thus, this patch makes caller manage
locking/unlocking.

[5/10] jffs2-xattr-v5.1-05-update_xattr_gc.patch

Signed-off-by: KaiGai Kohei <kaigai@ak.jp.nec.com>