xfs: add a xfs_iext_update_extent helper

This helper is used to update an extent record based on the extent index,
and can be used to provide a level of abstractions between callers that
want to modify in-core extent records and the details of the extent list
implementation.

Also switch all users of the xfs_bmbt_set_all(xfs_iext_get_ext(...))
pattern to this new helper.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
diff --git a/fs/xfs/libxfs/xfs_inode_fork.c b/fs/xfs/libxfs/xfs_inode_fork.c
index 0e80f34..fb310d08 100644
--- a/fs/xfs/libxfs/xfs_inode_fork.c
+++ b/fs/xfs/libxfs/xfs_inode_fork.c
@@ -2023,3 +2023,15 @@ xfs_iext_get_extent(
 	xfs_bmbt_get_all(xfs_iext_get_ext(ifp, idx), gotp);
 	return true;
 }
+
+void
+xfs_iext_update_extent(
+	struct xfs_ifork	*ifp,
+	xfs_extnum_t		idx,
+	struct xfs_bmbt_irec	*gotp)
+{
+	ASSERT(idx >= 0);
+	ASSERT(idx < xfs_iext_count(ifp));
+
+	xfs_bmbt_set_all(xfs_iext_get_ext(ifp, idx), gotp);
+}