Commit b9684a71 ("block, loop: support partitions without scanning")
adds GD_SUPPRESS_PART_SCAN for replacing part function of
GENHD_FL_NO_PART. But looks blk_add_partitions() is missed, since
loop doesn't want to add partitions if GENHD_FL_NO_PART was set.
And it causes regression on libblockdev (as called from udisks) which
operates with the LO_FLAGS_PARTSCAN.

Fixes the issue by not adding partitions if GD_SUPPRESS_PART_SCAN is
set.

Fixes: b9684a71 ("block, loop: support partitions without scanning")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220823103819.395776-1-ming.lei@redhat.comSigned-off-by: Jens Axboe <axboe@kernel.dk>