ubi: Add six fault injection type for testing

This commit adds six fault injection type for testing to cover the
abnormal path of the UBI driver.

Inject the following faults when the UBI reads the LEB:
 +----------------------------+-----------------------------------+
 |    Interface name          |       emulate behavior            |
 +----------------------------+-----------------------------------+
 |  emulate_eccerr            | ECC error                         |
 +----------------------------+-----------------------------------+
 |  emulate_read_failure      | read failure                      |
 |----------------------------+-----------------------------------+
 |  emulate_io_ff             | read content as all FF            |
 |----------------------------+-----------------------------------+
 |  emulate_io_ff_bitflips    | content FF with MTD err reported  |
 +----------------------------+-----------------------------------+
 |  emulate_bad_hdr           | bad leb header                    |
 |----------------------------+-----------------------------------+
 |  emulate_bad_hdr_ebadmsg   | bad header with ECC err           |
 +----------------------------+-----------------------------------+

Signed-off-by: ZhaoLong Wang <wangzhaolong1@huawei.com>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>