fix #2641: allow mounting of CIFS subdirectories

CIFS/SMB supports directly mounting subdirectories, so it makes sense to
also allow the --subdir parameter for these storages. The subdir
parameter was moved from CephFSPlugin.pm to Plugin.pm, because it isn't
specific to CephFS anymore.

Signed-off-by: Leo Nunner <l.nunner@proxmox.com>

PVE/Storage/Plugin.pm

@@ -169,6 +169,11 @@ my $defaultData = {
 	    type => 'boolean',
 	    optional => 1,
 	},
+	subdir => {
+	    description => "Subdir to mount.",
+	    type => 'string', format => 'pve-storage-path',
+	    optional => 1,
+	},
 	'format' => {
 	    description => "Default image format.",
 	    type => 'string', format => 'pve-storage-format',