lib/nfs/Base: kludge to reduce number of NFS mounts

Creating a NfsStorage sets its own export_name as the "base".  Now
NfsFileReader can use this information to derive the export_name to be
mounted, instead of guessing.  This solves the "too many connection"
problem on the NFS server while updating the database.

src/storage/plugins/NfsStorage.cxx

@@ -23,6 +23,7 @@
 #include "storage/StorageInterface.hxx"
 #include "storage/FileInfo.hxx"
 #include "lib/nfs/Domain.hxx"
+#include "lib/nfs/Base.hxx"
 #include "fs/AllocatedPath.hxx"
 #include "util/Error.hxx"
 #include "thread/Mutex.hxx"
@@ -256,6 +257,8 @@ CreateNfsStorageURI(const char *base, Error &error)
 		return nullptr;
 	}
 
+	nfs_set_base(server.c_str(), mount);
+
 	return new NfsStorage(base, ctx);
 }