Kernel+FileManager: Disallow watch_file() in unsupported file systems

Currently only Ext2FS and TmpFS supports InodeWatchers. We now fail
with ENOTSUPP if watch_file() is called on e.g ProcFS.

This fixes an issue with FileManager chewing up all the CPU when /proc
was opened. Watchers don't keep the watched Inode open, and when they
close, the watcher FD will EOF.

Since nothing else kept /proc open in FileManager, the watchers created
for it would EOF immediately, causing a refresh over and over.

Fixes #879.

Kernel/FileSystem/Ext2FileSystem.h

@@ -74,6 +74,8 @@ public:
 
     virtual KResult prepare_to_unmount() const override;
 
+    virtual bool supports_watchers() const override { return true; }
+
 private:
     typedef unsigned BlockIndex;
     typedef unsigned GroupIndex;