UBIFS: clean up free space checking
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 <19b5733dcea388885746d36043d3568bba5b4df7@nokia.com>
Reviewed-by: Adrian Hunter <026f70fb9023141549652ea6fafcf9ec193f6274@nokia.com>

TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs