Claude review: drm/amdkfd: Implement interval tree for userptr ranges

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

**Overview:** Adds interval tree for efficient range lookup.

**Issues:**

1. **Locking Missing - Critical:**
```c
+static bool discard_invalid_ranges(struct kgd_mem *mem,
+				unsigned long inv_start, unsigned long inv_end)
+{
+	for (node = interval_tree_iter_first(&mem->user_ranges_itree, inv_start, inv_end - 1);
```
No lock assertion or documentation. The interval tree is being iterated without documented protection. If ranges can be added/removed concurrently, this could crash.

2. **Integer Underflow:**
```c
+	for (node = interval_tree_iter_first(&mem->user_ranges_itree, inv_start, inv_end - 1);
```
If `inv_end == 0`, this underflows to `ULONG_MAX`. Should validate `inv_end > inv_start`.

3. **Debug Logging in Hot Path:**
```c
+		pr_debug("Range [0x%llx-0x%llx) marked invalid\n",
```
Called from MMU notifier callback which is performance-critical. Debug logging here could impact performance.

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