e2fsck: configure one pfsck thread

e2fsck: configure one pfsck thread

This patch creates only one thread to do pass1 check if pthreads are
enabled. The same codes can be used to create multiple threads, but
other functions need to be modified to get ready for that.

E2fsprogs-commit: b2a9a40831dfbb6dacf8bbc819acac2c25ab6980

Change-Id: I3df998a8ecc00d2dc5e959f5a9991a6b65182572
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>