badblocks: fix wrong return value in badblocks_set if badblocks are disabled

MD's rdev_set_badblocks() expects that badblocks_set() returns 1 if
badblocks are disabled, otherwise, rdev_set_badblocks() will record
superblock changes and return success in that case and md will fail to
report an IO error which it should.

This bug has existed since badblocks were introduced in commit
9e0e252a ("badblocks: Add core badblock management code").

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Acked-by: Guoqing Jiang <gqjiang@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>