gfs2: Fix usage of bio->bi_status in gfs2_end_log_write

bio->bi_status is an index into the blk_errors array, not an errno. Its
__bitwise tag is cast away here, resulting in a sparse warning:

  fs/gfs2/lops.c:207:22: warning: cast from restricted blk_status_t

We could either add __force to the cast and continue logging bi_status
in the error message, or we could look up the errno in the array and log
that. As sdp->sd_log_error is used as an errno in all other cases, look
up the errno here for consistency.

Signed-off-by: Andrew Price <anprice@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>