xfs_repair: fix handling of data blocks colliding with existing metadata

xfs_repair: fix handling of data blocks colliding with existing metadata

Prior to commit a406779bc8d8, any blocks in a data fork extent that
collided with existing blocks would cause the entire data fork extent to
be rejected.  Unfortunately, the patch to add data block sharing support
suppressed checking for any collision, including metadata.  What we
really wanted to do here during a check_dups==1 scan is to is check for
specific collisions and without updating the block mapping data.

So, move the check_dups test after the for-switch construction.  This
re-enables detecting collisions between data fork blocks and a
previously scanned chunk of metadata, and improves the specificity of
the error message that results.

This was found by fuzzing recs[2].free=zeroes in xfs/364, though this
patch alone does not solve all the problems that scenario presents.

Fixes: a406779bc8d8 ("xfs_repair: handle multiple owners of data blocks")
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>