Fix bug in the unmapping code

Previously trying to funmap an address after the start of the grant
would result in an overflow only caught when compiling the kernel with
debug assertions enabled.

src/context/memory.rs

@@ -521,11 +521,32 @@ impl AddrSpace {
             let intersection = conflicting_span.intersection(requested_span);
 
             requested_span = {
-                let offset = conflicting_span.base.offset_from(requested_span.base);
-                PageSpan::new(
-                    conflicting_span.end(),
-                    requested_span.count - offset - conflicting_span.count,
-                )
+                // In the following diagrams [---> indicates a range of
+                // base..base+count where the [ is at the base and > is at
+                // base+count. In other words, the [ is part of the range and
+                // the > is not part of the range.
+                if conflicting_span.end() < requested_span.end() {
+                    // [------>     conflicting_span
+                    //    [-------> requested_span
+                    //        [---> next requested_span
+                    // or
+                    //    [---->    conflicting_span
+                    // [----------> requested_span
+                    //         [--> next requested_span
+                    PageSpan::new(
+                        conflicting_span.end(),
+                        requested_span.end().offset_from(conflicting_span.end()),
+                    )
+                } else {
+                    // [----------> conflicting_span
+                    //    [----->   requested_span
+                    //              next requested_span
+                    // or
+                    //   [--------> conflicting_span
+                    // [-------->   requested_span
+                    //              next requested_span
+                    PageSpan::empty()
+                }
             };
 
             let (before, grant, after) = grant
@@ -649,6 +670,9 @@ impl PageSpan {
     pub fn new(base: Page, count: usize) -> Self {
         Self { base, count }
     }
+    pub fn empty() -> Self {
+        Self { base: Page::containing_address(VirtualAddress::new(0)), count: 0 }
+    }
     pub fn validate_nonempty(address: VirtualAddress, size: usize) -> Option<Self> {
         Self::validate(address, size).filter(|this| !this.is_empty())
     }