LU-3230 osp: unstick precreates on unmount

LU-3230 osp: unstick precreates on unmount

Allow osp_precreate_reserve to propagate error, Add debug code to
illuminate what the error code is if not EIO.

Test-Parameters: mdsfilesystemtype=zfs mdtfilesystemtype=zfs ostfilesystemtype=zfs
Test-Parameters: mdsfilesystemtype=zfs mdtfilesystemtype=zfs ostfilesystemtype=zfs mdscount=3
Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Change-Id: I1c15375d5e50d5a47a72b6c3c8e27512f51c3f9c
Reviewed-on: http://review.whamcloud.com/7995
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Johann Lombardi <johann.lombardi@intel.com>