cygwin: readdir: don't lookup mount target inodes

cygwin: readdir: don't lookup mount target inodes

So far Cygwin's readdir returned the inode number of a mount target
in d_ino, rather than the actual inode number of the mount point in
the underlying filesystem.  This not only results in a performance
hit if the mount target is a remote FS, it is also not done on other
POSIX systems.

Remove the code evaluating the mount target inode number.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>