Kernel: Make InodeWatcher inode registration completely OOM-fallible

InodeWatcher::register_inode was already partially fallible, but the
insertion of the inodes and watch descriptions into their respective
hash maps was not. Note that we cannot simply TRY the insertion into
both, as that could result in an inconsistent state, instead we must
remove the inode from the inode hash map if the insertion into the
watch description hash map failed.

Kernel/FileSystem/InodeWatcher.cpp

@@ -123,8 +123,12 @@ ErrorOr<int> InodeWatcher::register_inode(Inode& inode, unsigned event_mask)
 
     auto description = TRY(WatchDescription::create(wd, inode, event_mask));
 
-    m_inode_to_watches.set(inode.identifier(), description.ptr());
-    m_wd_to_watches.set(wd, move(description));
+    TRY(m_inode_to_watches.try_set(inode.identifier(), description.ptr()));
+    auto result = m_wd_to_watches.try_set(wd, move(description));
+    if (result.is_error()) {
+        m_inode_to_watches.remove(inode.identifier());
+        return result.release_error();
+    }
 
     inode.register_watcher({}, *this);
     return wd;