mkfs.ubifs: Improve error handling of is_contained()

mkfs.ubifs: Improve error handling of is_contained()

The is_contained() function returns -1 if an error occurs when
canonicalizing the output file path/root directory. This resulted in the
confusing error message 'Error: The output file cannot be in the UBIFS
root' when specifying a non-existent directory for the output.
This patch changes the error handling to display a different error
message for the case when is_contained() returns -1.
Additionally it frees all memory allocated by is_contained().

Signed-off-by: Marcus Prebble <marcus.prebble@axis.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>