Kernel: Fix race condition in TmpFSInode::notify_watchers()

We were doing this dance in notify_watchers():

    set_metadata_dirty(true);
    set_metadata_dirty(false);

This was done in order to force out inode watcher events immediately.
Unfortunately, this was racy, as if SyncTask got scheduled at the wrong
moment, it would try to flush metadata for a clean inode. This then got
trapped by the VERIFY() statement in Inode::sync_all():

    VERIFY(inode.is_metadata_dirty());

This patch fixes the issue by replacing notify_watchers() with lazy
metadata notifications like all other filesystems.

Kernel/FileSystem/TmpFS.cpp

@@ -176,7 +176,7 @@ ErrorOr<size_t> TmpFSInode::write_bytes(off_t offset, size_t size, const UserOrK
             m_content = move(tmp);
         }
         m_metadata.size = new_size;
-        notify_watchers();
+        set_metadata_dirty(true);
     }
 
     TRY(buffer.read(m_content->data() + offset, size)); // TODO: partial reads?
@@ -210,12 +210,6 @@ TmpFSInode::Child* TmpFSInode::find_child_by_name(StringView name)
     return nullptr;
 }
 
-void TmpFSInode::notify_watchers()
-{
-    set_metadata_dirty(true);
-    set_metadata_dirty(false);
-}
-
 ErrorOr<void> TmpFSInode::flush_metadata()
 {
     // We don't really have any metadata that could become dirty.
@@ -232,7 +226,7 @@ ErrorOr<void> TmpFSInode::chmod(mode_t mode)
     MutexLocker locker(m_inode_lock);
 
     m_metadata.mode = mode;
-    notify_watchers();
+    set_metadata_dirty(true);
     return {};
 }
 
@@ -242,7 +236,7 @@ ErrorOr<void> TmpFSInode::chown(UserID uid, GroupID gid)
 
     m_metadata.uid = uid;
     m_metadata.gid = gid;
-    notify_watchers();
+    set_metadata_dirty(true);
     return {};
 }
 
@@ -332,7 +326,7 @@ ErrorOr<void> TmpFSInode::truncate(u64 size)
     }
 
     m_metadata.size = size;
-    notify_watchers();
+    set_metadata_dirty(true);
     return {};
 }
 
@@ -341,7 +335,7 @@ ErrorOr<void> TmpFSInode::set_atime(time_t time)
     MutexLocker locker(m_inode_lock);
 
     m_metadata.atime = time;
-    notify_watchers();
+    set_metadata_dirty(true);
     return {};
 }
 
@@ -350,7 +344,7 @@ ErrorOr<void> TmpFSInode::set_ctime(time_t time)
     MutexLocker locker(m_inode_lock);
 
     m_metadata.ctime = time;
-    notify_watchers();
+    set_metadata_dirty(true);
     return {};
 }
 
@@ -359,7 +353,7 @@ ErrorOr<void> TmpFSInode::set_mtime(time_t t)
     MutexLocker locker(m_inode_lock);
 
     m_metadata.mtime = t;
-    notify_watchers();
+    set_metadata_dirty(true);
     return {};
 }