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.

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