xfs_repair: handle negative errors from read

xfs_repair: handle negative errors from read

xfs_repair does not handle read() errors while searching for secondary
superblocks. This problem is identified with a simple test case:

 (1) delete primary superblock of xfs partition with
        #dd if=/dev/zero of=/dev/sda1 bs=512 count=1
        #sync
 (2) run xfs_repair, and remove the storage  while it is searching for
     secondary superblocks

xfs_repair will loop forever, printing ............

Signed-off-by: Ajeet Yadav <ajeet.yadav.77@gmail.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>