vfs: Fix "emptydir" mount option

[olce]

Summary

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).

Test Plan

Manually tested (UFS, ZFS).

[kib]

I slightly prefer the previous way of loop organization, where the end label and gotos are not used, but I do not insist.

sys/kern/vfs_subr.c

6412

The cast is redundant.

6430

Style requires multi-line comments to be of the form

/*
 * Comment text
 * more of it
 */

[olce]

Comments addressed.

[kib]

sys/kern/vfs_subr.c

6444

Still the wrong style (I did not marked each style violation, only some instances).

[olce]

Comments addressed.

[olce]

Move a comment

[kib]

Please mail me all three patches formatted with git, and with right metadata: author, commit summary, and tag for the corresponding review ('Differential revision: <url>'). Thank you.