xfs: simplify inode to transaction joining

Currently we need to either call IHOLD or xfs_trans_ihold on an inode when
joining it to a transaction via xfs_trans_ijoin.

This patches instead makes xfs_trans_ijoin usable on it's own by doing
an implicity xfs_trans_ihold, which also allows us to drop the third
argument.  For the case where we want to hold a reference on the inode
a xfs_trans_ijoin_ref wrapper is added which does the IHOLD and marks
the inode for needing an xfs_iput.  In addition to the cleaner interface
to the caller this also simplifies the implementation.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>

diff --git a/fs/xfs/linux-2.6/xfs_super.c b/fs/xfs/linux-2.6/xfs_super.c
index 4b90e4b..b8ad17e 100644
--- a/fs/xfs/linux-2.6/xfs_super.c
+++ b/fs/xfs/linux-2.6/xfs_super.c
@@ -1023,8 +1023,7 @@
 	 * an inode in another recent transaction.  So we play it safe and
 	 * fire off the transaction anyway.
 	 */
-	xfs_trans_ijoin(tp, ip, XFS_ILOCK_EXCL);
-	xfs_trans_ihold(tp, ip);
+	xfs_trans_ijoin(tp, ip);
 	xfs_trans_log_inode(tp, ip, XFS_ILOG_CORE);
 	xfs_trans_set_sync(tp);
 	error = xfs_trans_commit(tp, 0);