UBIFS: fully sort GCed nodes

The 'joinup()' function cannot deal with situations when nodes
go in reverse order - it just leaves them in this order. This
patch implement full nodes sorting using n*log(n) algorithm.
It sorts data nodes for bulk-read, and direntry nodes for
readdir().

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>