LU-3273 mdt: Hold med_open_lock before walking med_open_head

Fixed a bug where during replay mdt_mfd_open() calls mdt_handle2mfd()
without acquiring the med_open_lock.
We now take the med_open_lock before traversing med_open_head list.
This bug was noticed during the analysis of LU-3233.

Signed-off-by: Swapnil Pimpale <spimpale@ddn.com>
Change-Id: Ib879f65d41d35f266897e8961dac78e6c4f0d9ec
Reviewed-on: http://review.whamcloud.com/7272
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>