Use _Noreturn (C11; GNU89) properly

A function that returns with no value is a different thing from a
function that doesn't return at all.  Those are two orthogonal
concepts, commonly confused.

pthread_create(3) expects a pointer to a start routine that has a
very precise prototype:

    void *(*start_routine)(void *);

However, other thread functions, such as kernel ones, expect:

    void (*start_routine)(void *);

Providing a different one is incorrect, and has only been working
because the ABIs happen to produce a compatible function.

We should use '_Noreturn void', since it's the natural type, and
then provide a '_Noreturn void *' wrapper for pthread functions.

For consistency, replace most cases of __NORETURN or
__attribute__((noreturn)) by _Noreturn.  _Noreturn is understood
by -std=gnu89, so it should be safe to use everywhere.

Ref: https://github.com/openzfs/zfs/pull/13110#discussion_r808450136
Ref: https://software.codidact.com/posts/285972
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Co-authored-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>
Closes #13120

module/zfs/mmp.c

@@ -187,7 +187,7 @@ uint_t zfs_multihost_import_intervals = MMP_DEFAULT_IMPORT_INTERVALS;
 uint_t zfs_multihost_fail_intervals = MMP_DEFAULT_FAIL_INTERVALS;
 
 static void *const mmp_tag = "mmp_write_uberblock";
-static void mmp_thread(void *arg);
+static _Noreturn void mmp_thread(void *arg);
 
 void
 mmp_init(spa_t *spa)
@@ -217,7 +217,7 @@ mmp_thread_enter(mmp_thread_t *mmp, callb_cpr_t *cpr)
 	mutex_enter(&mmp->mmp_thread_lock);
 }
 
-static void
+static _Noreturn void
 mmp_thread_exit(mmp_thread_t *mmp, kthread_t **mpp, callb_cpr_t *cpr)
 {
 	ASSERT(*mpp != NULL);
@@ -537,7 +537,7 @@ mmp_write_uberblock(spa_t *spa)
 	zio_nowait(zio);
 }
 
-static void
+static _Noreturn void
 mmp_thread(void *arg)
 {
 	spa_t *spa = (spa_t *)arg;