fs/attr.c: is_sxid can be boolean

This patch makes is_sxid return bool to improve readability
due to this particular function only using either one or zero
as its return value.

No functional change.

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