vfs: Export get_next_dirent() as vn_dir_next_dirent()

Move internal-to-'vfs_default.c' get_next_dirent() to 'vfs_vnops.c' and
export it for use by other parts of the VFS. This is a preparatory
change for using it in vfs_emptydir().

No functional change.

Reviewed by: kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D39755