smb: Improve enumeration performance
The samba backend calls stat for each file during the enumeration job. Unfortunately, this is significantly slower with the recent samba versions. However, smbc_readdirplus2() has been recently added and it can be used to improve the performance as all the necessary info is transferred in one network request. Let's use smbc_readdirplus2() if available.
See: https://gitlab.com/samba-team/samba/merge_requests/302
Relates: #306
Edited by Ondrej Holy