Bring some updates from ufs_lookup to ext2fs.

r156418:

Don't set IN_CHANGE and IN_UPDATE on inodes for potentially suspended
file systems.  This could cause deadlocks when creating snapshots.
(We can't do snapshots on ext2fs but it is useful to keep things in sync).

r183079:

- Only set i_offset in the parent directory's i-node during a lookup for
  non-LOOKUP operations.
- Relax a VOP assertion for a DELETE lookup.

r187528:

Move the code from ufs_lookup.c used to do dotdot lookup, into
the helper function. It is supposed to be useful for any filesystem
that has to unlock dvp to walk to the ".." entry in lookup routine.

MFC after:	5 days