[CIFS] Fix UNC path prefix on QueryUnixPathInfo to have correct slash

When a share was in DFS and the server was Unix/Linux, we were sending paths of the form
    \\server\share/dir/file
rather than
    //server/share/dir/file

There was some discussion between me and jra over whether we should use
    /server/share/dir/file
as MS sometimes says - but the documentation for this claims it should be
doubleslash for this type of UNC-like path format and that works, so leaving
it as doubleslash but converting the \ to / in the the //server/share portion.

This gets Samba to now correctly return STATUS_PATH_NOT_COVERED when it is
supposed to (Windows already did since the direction of the slash was not an issue
for them).  Still need another minor change to fully enable DFS (need to finish
some chages to SMBGetDFSRefer

Signed-off-by: Steve French <sfrench@us.ibm.com>