smb: Improve batching when sending enumerate GFileInfos

Instead of creating a list of GFileInfos and sending them when the
entire directory has been read, call g_vfs_job_enumerate_add_info
to send them in batches (default of 50).  This simplifies the code
and should reduce the memory usage when enumerating a large directory.

https://bugzilla.gnome.org/show_bug.cgi?id=730856