Kernel: Make Inode::lookup() return a RefPtr<Inode>

Previously this API would return an InodeIdentifier, which meant that
there was a race in path resolution where an inode could be unlinked
in between finding the InodeIdentifier for a path component, and
actually resolving that to an Inode object.

Attaching a test that would quickly trip an assertion before.

Test: Kernel/path-resolution-race.cpp

Tests/Kernel/path-resolution-race.cpp

@@ -0,0 +1,16 @@
+#include <unistd.h>
+#include <sys/stat.h>
+
+int main()
+{
+    if (!fork()) {
+        for (;;) {
+            mkdir("/tmp/x", 0666);
+            rmdir("/tmp/x");
+        }
+    }
+    for (;;) {
+        chdir("/tmp/x");
+    }
+    return 0;
+}