md/bitmap: clear bitmap if bitmap_create failed

If bitmap_create returns an error, we need to call
either bitmap_destroy or bitmap_free to do clean up,
and the selection is based on mddev->bitmap is set
or not.

And the sysfs_put(bitmap->sysfs_can_clear) is moved
from bitmap_destroy to bitmap_free, and the comment
of bitmap_create is changed as well.

Signed-off-by: Guoqing Jiang <gqjiang@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>