xfs_scrub: make phase 4 go straight to fstrim if nothing to fix

xfs_scrub: make phase 4 go straight to fstrim if nothing to fix

If there's nothing to repair in phase 4, there's no need to hold up the
FITRIM call to do the summary count scan that prepares us to repair
filesystem metadata.  Rearrange this a bit.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
[sandeen: fix unfixable_errors test logic thinko]
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>