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/Process.cpp

@@ -3230,6 +3230,9 @@ int Process::sys$watch_file(const char* path, int path_length)
     auto& custody = custody_or_error.value();
     auto& inode = custody->inode();
 
+    if (!inode.fs().supports_watchers())
+        return -ENOTSUP;
+
     int fd = alloc_fd();
     if (fd < 0)
         return fd;