xfs: ioctl check for capabilities in the current user namespace

Use inode_capable() to check if SUID|SGID bits should be cleared to match
similar check in inode_change_ok().

The check for CAP_LINUX_IMMUTABLE was not modified since all other file
systems also check against init_user_ns rather than current_user_ns.

Only allow changing of projid from init_user_ns.

Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Gao feng <gaofeng@cn.fujitsu.com>
Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
Signed-off-by: Ben Myers <bpm@sgi.com>

kernel/capability.c

@@ -464,3 +464,4 @@ bool inode_capable(const struct inode *inode, int cap)
 
 	return ns_capable(ns, cap) && kuid_has_mapping(ns, inode->i_uid);
 }
+EXPORT_SYMBOL(inode_capable);