xfs_scrub: fall back to scrub-by-handle if opening handles fails

xfs_scrub: fall back to scrub-by-handle if opening handles fails

Back when I originally wrote xfs_scrub, I decided that phase 3 (the file
scrubber) should try to open all regular files by handle to pin the file
during the scrub.  At the time, I decided that an ESTALE return value
should cause the entire inode group (aka an inobt record) to be
rescanned for thoroughness reasons.

Over the past four years, I've realized that checking the return value
here isn't necessary.  For most runtime errors, we already fall back to
scrubbing with the file handle, at a fairly small performance cost.

For ESTALE, if the file has been freed and reallocated, its metadata has
been rewritten completely since bulkstat, so it's not necessary to check
it for latent disk errors.  If the file was freed, we can simply move on
to the next file.  If the filesystem is corrupt enough that
open-by-handle fails (this also results in ESTALE), we actually /want/
to fall back to scrubbing that file by handle, not rescanning the entire
inode group.

Therefore, remove the ESTALE check and leave a comment detailing these
findings.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>