yaffs: Fix yaffs_readdir when dereferencing a symlink

If the directory is a symlink then follow the symlink.

eg
yaffs_mount("/nand/");
yaffs_symlink("x", "/nand/sym");
yaffs_mkdir("/nand/x",0666);

After that:

yaffs_opendir("/nand/sym/") works, but opendir("/nand/sym") fails because the
symlink itself is found. Fix by dereferencing the symlink in yaffs_opendir().

Thanks to Trent Lillehaugen for finding that.

Signed-off-by: Charles Manning <cdhmanning@gmail.com>