Btrfs: do not log extents when we only log new names

When we log new names, we need to log just enough to recreate the inode
during log replay, and there is no need to log extents along with it.

This actually fixes a bug revealed by xfstests 241, where it shows
that we're logging some extents that have not updated metadata,
so we don't get proper EXTENT_DATA items to be copied to log tree.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>