Fix vfs_emptydir(). It would consider directories containing directories
with name of the form 'X.' (X being any authorized byte) as empty. Also,
it would cause VOP_READDIR() to return an error on directories
containing enough whiteouts. While here, use a more decently sized
buffer as done elsewhere.
Remove ad-hoc iteration on the directory's content and instead use the
newly exported vn_dir_next_dirent() function (this is what fixes the
second problem mentioned above).