LU-1206: mdt: Fix error handling in mdt_mfd_open

LU-1206: mdt: Fix error handling in mdt_mfd_open

In mdt_mfd_open if the mo_open() call failed or we could not allocate
mfd, we also need to undo write/exec reference count in order to
not mess up with subsequent exec/write accesses.

Signed-off-by: Prakash Surya <surya1@llnl.gov>
Signed-off-by: Oleg Drokin <green@whamcloud.com>
Change-Id: I3bd98bd68368b48f2afaa7bb450d3a9947c992ac
Reviewed-on: http://review.whamcloud.com/2300
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>