Claude review: drm/atomic: Allocate atomic_state at the beginning of atomic_ioctl

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

Patch Review

This moves `drm_atomic_state_alloc()` and `drm_modeset_acquire_init()` before the early validation checks, converting early `return -EINVAL` to `goto out`. This means every trivially-invalid call now allocates and frees an atomic state, acquires/releases modeset locks, etc. That's unnecessary overhead for simple validation failures.

The original code was structured to fail fast cheaply. The justification is to have `state->error_code` available, but for these trivial "invalid flag" / "reserved field set" cases, the error is already clear from the errno. Consider only allocating early when the error reporting pointer (`arg->reserved`) is actually provided.

**The `arg->reserved` check is removed** from the validation:
```c
-	if (arg->reserved) {
-		drm_dbg_atomic(dev, "commit failed: reserved field set\n");
-		return -EINVAL;
-	}
```
This is the mechanism that ensures forward/backward compatibility. Removing it without a versioning scheme means old kernels that don't understand the new meaning of `reserved` will reject the call, which is fine, but old userspace that sets `reserved` to garbage will now have `copy_to_user` write to a random address. The capability check in patch 7 partially addresses this (new userspace should check the cap first), but the kernel should still validate the pointer somehow.

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