fs/block_dev.c: make sb_is_blkdev_sb return bool when CONFIG_BLOCK undefined

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: Yaowei Bai <baiyaowei@cmss.chinamobile.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>