UBIFS: amend ubifs_recover_leb interface

Instead of passing "grouped" parameter to 'ubifs_recover_leb()' which tells
whether the nodes are grouped in the LEB to recover, pass the journal head
number and let 'ubifs_recover_leb()' look at the journal head's 'grouped' flag.

This patch is a preparation to a further fix where we'll need to know the
journal head number for other purposes.

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