LU-6768 osd: unmap reallocated blocks

LU-6768 osd: unmap reallocated blocks

call unmap_underlying_metadata() on every allocated
block. otherwise metadata blocks released in a previous
transaction can be written by the kernel corrupting
user's data. pblock != 0 should be a good sign of that.

Change-Id: I0d06611feb384a3f7ef5d8e5b34951822369ed0f
Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-on: http://review.whamcloud.com/15593
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>