ocfs2: reflink: fix slow unlink for refcounted file

When running ocfs2 test suite multiple nodes reflink stress test, for a
4 nodes cluster, every unlink() for refcounted file needs about 700s.

The slow unlink is caused by the contention of refcount tree lock since
all nodes are unlink files using the same refcount tree.  When the
unlinking file have many extents(over 1600 in our test), most of the
extents has refcounted flag set.  In ocfs2_commit_truncate(), it will
execute the following call trace for every extents.  This means it needs
get and released refcount tree lock about 1600 times.  And when several
nodes are do this at the same time, the performance will be very low.

  ocfs2_remove_btree_range()
  --  ocfs2_lock_refcount_tree()
  ----  ocfs2_refcount_lock()
  ------  __ocfs2_cluster_lock()

ocfs2_refcount_lock() is costly, move it to ocfs2_commit_truncate() to
do lock/unlock once can improve a lot performance.

Signed-off-by: Junxiao Bi <junxiao.bi@oracle.com>
Cc: Wengang <wen.gang.wang@oracle.com>
Reviewed-by: Mark Fasheh <mfasheh@suse.de>
Cc: Joel Becker <jlbec@evilplan.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>