e2fsck: serialize fix operations

e2fsck: serialize fix operations

Allow different threads to fix at the same time could
be dangerous and error-prone now, and most of time
parallel scanning and checking is important.

So this patch adds a mutex to serialize
fix operations during pass1.

And the good benefit of this, we don't need block
allocations and free, superblock updates protection
any more, since only fix operations during pass1
could touch them.

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