UBIFS: clean up free space checking

recovery.c has 'is_empty()' helper and it is better to use
this helper instead of re-implementing it in several places.
This patch does this and removes some amount of unneeded code.

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