vfs: Fix "emptydir" mount option

vfs: Fix "emptydir" mount option

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).

PR:	270988
Reviewed by:	kib
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D39775