LU-376 Positive LL_DIR_END_OFF to indicate the tail of dir hash/offset

LU-376 Positive LL_DIR_END_OFF to indicate the tail of dir hash/offset

1) Keep 'MDS_DIR_END_OFF' unchanged (0xfffffffffffffffeULL) to simplify
   dir hash/offset related interoperability issues.
2) Introduce positive "LL_DIR_END_OFF" (0x7fffffffffffffffULL) on client
   to indicate the tail of dir hash/offset for up layer callers, like
   llseek(), readdir(), and so on.
3) Support new 2.x client to talk with old 2.0 server with 32bit hash.

Change-Id: I74c148482f91ed91dfffd608f98971566f1257ad
Signed-off-by: nasf <yong.fan@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/886
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Hudson
Reviewed-by: Oleg Drokin <green@whamcloud.com>