bs_sheepdog.c: fix a compiler warning due to an unused label

This gets rid of the following gcc warning in sd_open function by
removing the unused label 'retry':

 bs_sheepdog.c: In function 'sd_open':
 bs_sheepdog.c:1199:1: warning: label 'retry' defined but not used [-Wunused-label]
  retry:
  ^

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>