ubifs: Remove unreachable code in dbg_check_ltab_lnum

Because there is no break statement in the dead loop above,it is
impossible to execute the 'err=0' statement.Delete the code that
will never execute.

Fixes: 6fb324a4

 ("UBIFS: allocate ltab checking buffer on demand")
Signed-off-by: Kunwu Chan <chentao@kylinos.cn>
Cc: Kunwu Chan <kunwu.chan@hotmail.com>
Suggested-by: Richard Weinberger <richard.weinberger@gmail.com>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>