LU-957 scrub: Ancillary work for LFSCK/OI scrub

LU-957 scrub: Ancillary work for LFSCK/OI scrub

1) New debug sub-system - 'D_LFSCK'
For Lustre fsck/scrub running trace.

2) New MDT mount option - 'noscrub'
To disable auto triggering OI scrub when MDT mounts up
or by RPC which accesses inconsistent OI mapping entry.

3) Any object which will be used when Lustre server mounts
   should not be added into OI files to guarantee that the
   Lustre server can start up even though OI files corrupt.

Signed-off-by: Fan Yong <yong.fan@whamcloud.com>
Change-Id: I8b1f1f1160db25ce05bd2d5688f9ff1507e3841b
Reviewed-on: http://review.whamcloud.com/2897
Tested-by: Hudson
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>