Check for ext3/ext4 fs, then check kernel version.

This reverts commit e95c9231.

Revert "Hack for potential ext3/ext4 corruption issue"

This reverts commit 91155b9d.

Leonid Yuriev authored Jan 04, 2015 and Howard Chu committed Jan 04, 2015

dn2id.c:
  + assertion for mdb_id2l_insert() result;
  - unnecessary assignment;

search.c:
  - unused variable 'first';
  + range-check for mdb_idl_search() result;