LU-11457 osd-ldiskfs: scrub FID reuse

LU-11457 osd-ldiskfs: scrub FID reuse

It's possible that two inodes back point to the same FID, check
inodes in osd_scrub_check_update() to decide which mapping
should be kept:
* if one inode doesn't exist, its mapping is stale.
* if one inode mtime is after the other one, keep this mapping.
* if two inode mtimes equal, and one inode size is not 0, keep its
  mapping, otherwise two inode sizes are 0, just keep the existing
  mapping.

Remove IDIF support in osd_scrub_check_update() to simplify
code logic.

Add sanity-scrub 4e to verify it.

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: Ida020c2852c66f1a8910845bd16ab4c882858a4e