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 mdsfilesystemtype=zfs mdtfilesystemtype=zfs ostfilesystemtype=zfs
Test-Parameters: testlist=conf-sanity,conf-sanity,conf-sanity mdsfilesystemtype=ldiskfs mdtfilesystemtype=ldiskfs ostfilesystemtype=ldiskfs
Change-Id: I51c5fcc5d9e258b536e5d2f070eb9ca6cd67d5aa
Signed-off-by: Li Wei <wei.g.li@intel.com>
Reviewed-on: http://review.whamcloud.com/12319
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Liang Zhen <liang.zhen@intel.com>
Reviewed-by: Johann Lombardi <johann.lombardi@intel.com>