./.. changed during execution of find

[Pierre A. Humblet]

The case reported on the list is probably due to a change in chdir.
find scans / and chdir bin.
/bin is c:/cygwin/bin, but the canonical Posix name of that 
directory (longest prefix match) is /usr/bin.
Eventually find chdir .. and finds itself in /usr, not 
in /

Fixing that means stop using mount_info->conv_to_posix
and going back to normalize_posix_path.

Pierre