xfs_metadump: obfuscate remote symlinks on CRC filesystems

xfs_metadump: obfuscate remote symlinks on CRC filesystems

On CRC filesystems, the symlink block starts with a header,
which contains magic, "XLSM"

The code happens to "work" today w/o corrupting anything,
because it seems "XSLM" as a string, decides it's too short
to obfuscate, and leaves it alone.

But the real symlink target is untouched.  Fix that by moving
the pointer to the string we want to obfuscate by the size
of the header, and shorten the length to obfuscate accordingly.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>