Make align_up and align_down const

We can't make VirtAddr and PhysAddr methods const as they wrap an into
impl.

src/addr.rs

@@ -534,8 +534,12 @@ impl Sub<PhysAddr> for PhysAddr {
 /// Returns the greatest x with alignment `align` so that x <= addr. The alignment must be
 ///  a power of 2.
 #[inline]
-pub fn align_down(addr: u64, align: u64) -> u64 {
+pub const fn align_down(addr: u64, align: u64) -> u64 {
+    #[cfg(feature = "const_fn")]
     assert!(align.is_power_of_two(), "`align` must be a power of two");
+    #[cfg(not(feature = "const_fn"))]
+    [(); 1][!align.is_power_of_two() as usize];
+
     addr & !(align - 1)
 }
 
@@ -544,8 +548,12 @@ pub fn align_down(addr: u64, align: u64) -> u64 {
 /// Returns the smallest x with alignment `align` so that x >= addr. The alignment must be
 /// a power of 2.
 #[inline]
-pub fn align_up(addr: u64, align: u64) -> u64 {
+pub const fn align_up(addr: u64, align: u64) -> u64 {
+    #[cfg(feature = "const_fn")]
     assert!(align.is_power_of_two(), "`align` must be a power of two");
+    #[cfg(not(feature = "const_fn"))]
+    [(); 1][!align.is_power_of_two() as usize];
+
     let align_mask = align - 1;
     if addr & align_mask == 0 {
         addr // already aligned