fs: aio ioprio add explicit block layer dependence

Previously, the ioprio_check_cap function was only defined when CONFIG_BLOCK
was set. Make this relationship explicit and add a stub for !CONFIG_BLOCK.
Signed-off-by: Adam Manzanares <adam.manzanares@wdc.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

include/linux/ioprio.h

@@ -77,6 +77,13 @@ extern int ioprio_best(unsigned short aprio, unsigned short bprio);
 
 extern int set_task_ioprio(struct task_struct *task, int ioprio);
 
+#ifdef CONFIG_BLOCK
 extern int ioprio_check_cap(int ioprio);
+#else
+static inline int ioprio_check_cap(int ioprio)
+{
+	return -ENOTBLK;
+}
+#endif /* CONFIG_BLOCK */
 
 #endif