e2fsck: add start/end group for thread

e2fsck: add start/end group for thread

When multi-threads are used for check, each thread needs to jump
to different group in pass1 check. This patch adds the group
jumping support. But still, only one thread is used to check.

E2fsprogs-commit: f64ff7f53c6fb22c7e4bebe86749b1be31439424

Change-Id: I904bedcbd02bca591665943cff5f2d7a6fa50fbe
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: Theodore Ts'o <tytso@mit.edu>