UBIFS: simplify returns

Directly return recover_head() and ubifs_leb_unmap()
instead of storing value in err and testing it.

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>