Kernel: Move Blocker setup out from constructors into setup_blocker()

Instead of registering with blocker sets and whatnot in the various
Blocker subclass constructors, this patch moves such initialization
to a separate setup_blocker() virtual.

setup_blocker() returns false if there's no need to actually block
the thread. This allows us to bail earlier in Thread::block().

Kernel/FileSystem/Plan9FileSystem.cpp

@@ -425,6 +425,11 @@ bool Plan9FS::Blocker::unblock(u16 tag)
     return unblock();
 }
 
+bool Plan9FS::Blocker::setup_blocker()
+{
+    return add_to_blocker_set(m_fs.m_completion_blocker);
+}
+
 void Plan9FS::Blocker::will_unblock_immediately_without_blocking(UnblockImmediatelyReason)
 {
     {