LU-5654 osd-ldiskfs: Handle holes in osd_ldiskfs_read()

Current osd_ldiskfs_read() incorrectly returns zero and leaves the
corresponding portion of the buffer untouched when a block to be read
is not allocated.

Test-Parameters: testlist=conf-sanity,conf-sanity,conf-sanity,conf-sanity,conf-sanity
Change-Id: I3044f771fccfdddf1fee0c931e8996cbdd8df0c8
Signed-off-by: Li Wei <wei.g.li@intel.com>
Reviewed-on: http://review.whamcloud.com/12145
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Johann Lombardi <johann.lombardi@intel.com>
Tested-by: Jenkins
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>