LU-8465 e2fsck: open io-channel when copying fs

LU-8465 e2fsck: open io-channel when copying fs

This patch also add writethrough flag to the thread io-channel.
When multiple threads write the same disk, we don't want the
data being saved in memory cache. This will be useful in the
future, but even without that flag, the tests can be passed too.

This patch also cleanup the io channel cache of the global
context. Otherwise, after pass1 step, the next steps would use
old data saved in the cache. And the cached data might have
already been overwritten in pass1.

Change-Id: Ie9d93a2193394ab12550da6141d8b08b9e519ef0
Signed-off-by: Li Xi <lixi@ddn.com>
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Reviewed-on: https://review.whamcloud.com/35763
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Andreas Dilger <adilger@whamcloud.com>