ubi: Fix permission display of the debugfs files

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I60L5U


CVE: NA

-----------------------------------------

Some interface files in debugfs support the read method
dfs_file_read(), but their rwx permissions is shown as
unreadable.

For example:

 # ls -l /sys/kernel/debug/ubi/ubi0/
 total 0
 --w------- 1 root root 0 Oct 22 16:26 chk_fastmap
 --w------- 1 root root 0 Oct 22 16:26 chk_gen
 --w------- 1 root root 0 Oct 22 16:26 chk_io
 -r-------- 1 root root 0 Oct 22 16:26 detailed_erase_block_info
 --w------- 1 root root 0 Oct 22 16:26 tst_disable_bgt
 --w------- 1 root root 0 Oct 22 16:26 tst_emulate_bitflips
 --w------- 1 root root 0 Oct 22 16:26 tst_emulate_io_failures
 --w------- 1 root root 0 Oct 22 16:26 tst_emulate_power_cut
 --w------- 1 root root 0 Oct 22 16:26 tst_emulate_power_cut_max
 --w------- 1 root root 0 Oct 22 16:26 tst_emulate_power_cut_min

It shows that these files do not have read permission,
but we can actually read their contents.

 # echo 1 > /sys/kernel/debug/ubi/ubi0/chk_io
 # cat /sys/kernel/debug/ubi/ubi0/chk_io
 1

This patch adds the read permission display for file that
support the read method.

Signed-off-by: ZhaoLong Wang <wangzhaolong1@huawei.com>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>