UBIFS: simplify LEB recovery function further

Further simplify 'ubifs_recover_leb()' by noticing that we have to call
'clean_buf()' in any case, and it is fine to call it if the offset is
aligned to 'c->min_io_size'. Thus, we do not have to call it separately
from every "if" - just call it once at the end.

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