io-wq: provide IO_WQ_* constants for IORING_REGISTER_IOWQ_MAX_WORKERS arg items

The items passed in the array pointed by the arg parameter
of IORING_REGISTER_IOWQ_MAX_WORKERS io_uring_register operation
carry certain semantics: they refer to different io-wq worker categories;
provide IO_WQ_* constants in the UAPI, so these categories can be referenced
in the user space code.

Suggested-by: Jens Axboe <[e-mail]>
Complements: 2e480058ddc21ec5 ("io-wq: provide a way to limit max number of workers")
Signed-off-by: Eugene Syromiatnikov <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jens Axboe <[e-mail]>

fs/io-wq.c

···
 #include <linux/rculist_nulls.h>
 #include <linux/cpu.h>
 #include <linux/tracehook.h>
+#include <uapi/linux/io_uring.h>
 
 #include "io-wq.h"
 
···
 int io_wq_max_workers(struct io_wq *wq, int *new_count)
 {
 	int i, node, prev = 0;
+
+	BUILD_BUG_ON((int) IO_WQ_ACCT_BOUND   != (int) IO_WQ_BOUND);
+	BUILD_BUG_ON((int) IO_WQ_ACCT_UNBOUND != (int) IO_WQ_UNBOUND);
+	BUILD_BUG_ON((int) IO_WQ_ACCT_NR      != 2);
 
 	for (i = 0; i < 2; i++) {
 		if (new_count[i] > task_rlimit(current, RLIMIT_NPROC))

include/uapi/linux/io_uring.h

···
 	IORING_REGISTER_IOWQ_AFF		= 17,
 	IORING_UNREGISTER_IOWQ_AFF		= 18,
 
-	/* set/get max number of workers */
+	/* set/get max number of io-wq workers */
 	IORING_REGISTER_IOWQ_MAX_WORKERS	= 19,
 
 	/* this goes last */
 	IORING_REGISTER_LAST
+};
+
+/* io-wq worker categories */
+enum {
+	IO_WQ_BOUND,
+	IO_WQ_UNBOUND,
 };
 
 /* deprecated, see struct io_uring_rsrc_update */