e2fsck: open io-channel when copying fs

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.

Signed-off-by: Li Xi <lixi@ddn.com>
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Signed-off-by: Saranya Muruganandam <saranyamohan@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>