rusticl: check the returned pointer of mmap

Prior Linux 4.17 MAP_FIXED_NOREPLACE might not be respected and might
return a pointer different than the requested one.

Fixes: da4de8d7e3 ("rusticl: add support for coarse-grain buffer SVM")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35286>

src/gallium/frontends/rusticl/core/context.rs

@@ -319,7 +319,7 @@ impl Context {
                 unreachable!("SVM supported only on Linux")
             }
 
-            let res = unsafe {
+            let mut res = unsafe {
                 mmap(
                     vma.get() as usize as *mut c_void,
                     size.get() as usize,
@@ -335,6 +335,14 @@ impl Context {
                 return Err(CL_OUT_OF_HOST_MEMORY);
             }
 
+            if res as usize != vma.get() as usize {
+                unsafe {
+                    let ret = munmap(res, size.get() as usize);
+                    debug_assert_eq!(0, ret);
+                }
+                res = ptr::null_mut();
+            }
+
             res.cast()
         } else {
             unsafe { alloc::alloc(layout) }.cast()