LU-3029 osd-ldiskfs: clear old FMODE_32BITHASH for readdir

On MDS, the RPC service thread's "osd_thread_info" will be reused
without completely reset when switches from one RPC processing to
another RPC processing.

Some old client may not claim "OBD_CONNECT_64BITHASH" when connect
to the MDS, then it will be regarded as 32-bit client. For readdir
RPC from such old client, the MDS will use 32-bit dirhash, the RPC
service thread's "osd_thread_info::oti_it_ea::oie_file::f_mode" is
set as "FMODE_32BITHASH", which will not be dropped until restart.

If some RPC service threads (for readdir) are "FMODE_32BITHASH",
but some NOT, then for new client, which support 64-bit dirhash,
it may get trouble when traverses large directroy as following:

1) The first readdir RPC is served by the RPC service thread1,
   which is marked as "FMODE_32BITHASH" because it ever served
   readdir RPC from old client. So the thread1 still generates
   32-bit dirhash (major hash only) for the new client readdir.

2) The new client triggers another readdir RPC for the same dir
   with the 32-bit dirhash as cookie, which was returned by the
   thread1.

3) This time, the readdir RPC is served by another RPC service
   thread2, which is NOT marked as "FMODE_32BITHASH". Then the
   thread2 explains the readdir cookie as 64-bit dirhash, It's
   wrong. So the thread2 can NOT locate the position correctly.

So we need to clear some fields in "osd_thread_info" to avoid
to be reused when switch from one RPC processing to another.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I3b9aaede0bccab900f1d198c7093b98f0fc48945
Reviewed-on: http://review.whamcloud.com/6138
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Hudson
Reviewed-by: Ned Bass <bass6@llnl.gov>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Prakash Surya <surya1@llnl.gov>
Tested-by: Maloo <whamcloud.maloo@gmail.com>