Currently when CONFIG_BLOCK is defined sb_is_blkdev_sb returns bool,
while when CONFIG_BLOCK is not defined it returns int. Let's keep
consistent to make sb_is_blkdev_sb return bool as well when CONFIG_BLOCK
isn't defined.

No functional change.
Signed-off-by: NYaowei Bai <baiyaowei@cmss.chinamobile.com>
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>