Btrfs: make sure we wait on logged extents when fsycning two subvols

commit 9dba8cf128ef98257ca719722280c9634e7e9dc7 upstream.

If we have two fsync()'s race on different subvols one will do all of its work
to get into the log_tree, wait on it's outstanding IO, and then allow the
log_tree to finish it's commit.  The problem is we were just free'ing that
subvols logged extents instead of waiting on them, so whoever lost the race
wouldn't really have their data on disk.  Fix this by waiting properly instead
of freeing the logged extents.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>