xfs_scrub: handle concurrent directory updates during name scan

xfs_scrub: handle concurrent directory updates during name scan

The name scanner in xfs_scrub cannot lock a namespace (dirent or xattr)
and the kernel does not provide a stable cursor interface, which means
that we can see the same byte sequence multiple times during a scan.
This isn't a confusing name error since the kernel enforces uniqueness
on the byte sequence, so all we need to do here is update the old entry.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Chandan Babu R <chandanrlinux@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>