LU-8465 e2fsck: copy context when using multi-thread fsck

LU-8465 e2fsck: copy context when using multi-thread fsck

This patch only copy the context to a new one when -m is enabled.
It doesn't actually start any thread. When pass1 test finishes,
the new context is copied back to the original context.

Since the signal handler only changes the original context, so
add global_ctx in "struct e2fsck_struct" and use that to check
whether there is any signal of canceling.

This patch handles the long jump properly so that all the existing
tests can be passed even the context has been copied. Otherwise,
test f_expisize_ea_del would fail when aborting.

Change-Id: I99bc621165ac7470bbd54550f8198b0a0b35feee
Signed-off-by: Li Xi <lixi@ddn.com>
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Reviewed-on: https://review.whamcloud.com/35690
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>