Linux 5.2 compat: rw_tryupgrade()

Commit torvalds/linux@46ad0840b has removed the architecture specific
rwsem source and headers leaving only the generic version.  As part
of this change the RWSEM_ACTIVE_READ_BIAS and RWSEM_ACTIVE_WRITE_BIAS
macros were moved to the private kernel/locking/rwsem.h header.
This results in a build failure because these macros were required
to implement the rw_tryupgrade() compatibility function.

In practice, this isn't a major problem because there are only a
few consumers of rw_tryupgrade() and because consumers of rw_tryupgrade
should be written to retry using rw_enter(RW_WRITER).

After auditing all of the callers only dmu_zfetch() was determined
not to perform a retry.  It has been updated in this commit to
resolve this issue.

That said, the rw_tryupgrade() functionality should be considered
for possible removal in a future release due to the difficultly
in supporting the interface.

Reviewed-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Reviewed-by: Chunwei Chen <tuxoko@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #8730

module/spl/spl-rwlock.c

@@ -85,7 +85,8 @@ __rwsem_tryupgrade(struct rw_semaphore *rwsem)
 	spl_rwsem_unlock_irqrestore(&rwsem->wait_lock, flags);
 	return (ret);
 }
-#elif defined(HAVE_RWSEM_ATOMIC_LONG_COUNT)
+#elif defined(RWSEM_ACTIVE_MASK)
+#if defined(HAVE_RWSEM_ATOMIC_LONG_COUNT)
 static int
 __rwsem_tryupgrade(struct rw_semaphore *rwsem)
 {
@@ -104,6 +105,13 @@ __rwsem_tryupgrade(struct rw_semaphore *rwsem)
 	return (val == SPL_RWSEM_SINGLE_READER_VALUE);
 }
 #endif
+#else
+static int
+__rwsem_tryupgrade(struct rw_semaphore *rwsem)
+{
+	return (0);
+}
+#endif
 
 int
 rwsem_tryupgrade(struct rw_semaphore *rwsem)