e2fsck: simplify e2fsck context merging codes

e2fsck: simplify e2fsck context merging codes

We tried to copy thread context to global context directly
and then copy back some saved variables before merging.

Since we have finished almost all necessary variables
in the e2fsck context, we could simplify codes, and
this could help us understand what is missing rather
than hide problems.

E2fsprogs-commit: 41c991af7ed7244da0fd799f63e4e6fd0d6c5b25

Change-Id: I450e3f5fb26c5c6fe8a075373bf538965f443d02
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>