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/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index 772f3e7..aeac002 100644
--- a/fs/xfs/xfs_iomap.c
+++ b/fs/xfs/xfs_iomap.c
@@ -329,8 +329,7 @@
 	if (error)
 		goto error1;
 
-	xfs_trans_ijoin(tp, ip, XFS_ILOCK_EXCL);
-	xfs_trans_ihold(tp, ip);
+	xfs_trans_ijoin(tp, ip);
 
 	bmapi_flag = XFS_BMAPI_WRITE;
 	if ((flags & BMAPI_DIRECT) && (offset < ip->i_size || extsz))
@@ -597,8 +596,7 @@
 				return XFS_ERROR(error);
 			}
 			xfs_ilock(ip, XFS_ILOCK_EXCL);
-			xfs_trans_ijoin(tp, ip, XFS_ILOCK_EXCL);
-			xfs_trans_ihold(tp, ip);
+			xfs_trans_ijoin(tp, ip);
 
 			xfs_bmap_init(&free_list, &first_block);
 
@@ -761,8 +759,7 @@
 		}
 
 		xfs_ilock(ip, XFS_ILOCK_EXCL);
-		xfs_trans_ijoin(tp, ip, XFS_ILOCK_EXCL);
-		xfs_trans_ihold(tp, ip);
+		xfs_trans_ijoin(tp, ip);
 
 		/*
 		 * Modify the unwritten extent state of the buffer.