block/ioctl.c and fs/partition/check.c: check value returned by add_partition()

Now that add_partition() has been aught to propagate errors, let's check them.

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Abdel Benamrouche <draconux@gmail.com>
Cc: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>