[kernel] Fix the mysterious paging bug!!

There has been a random bug (that occurs frequently outside the debugger
but rarely inside the debugger, of course) that seemed to be caused by
inconsistent page mappings. Sometimes loading an ELF would work. Other
times loading that same ELF, the loader would complain of missing
sections or invalid headers. Worse, occasionally program execution would
jump off into random memory for no reason I could see by examining the
disassembly. This issue has been plauging me FOR A YEAR and I've been
pulling my hair out trying to find it.

https://stackoverflow.com/a/28384866

Eventually this stack overflow answer to a different question about
INVLPG gave me a hint that the 'accessed' flag of page table entries
might not be set on pages even if they end up in the TLB.

Good riddance to this damned bug.

src/kernel/vm_space.cpp

@@ -250,10 +250,10 @@ vm_space::clear(const obj::vm_area &vma, uintptr_t offset, size_t count, bool fr
 
         if (flags & page_flags::present) {
             e = 0;
-            if (flags & page_flags::accessed) {
-                auto *addr = reinterpret_cast<const uint8_t *>(it.vaddress());
-                asm ( "invlpg %0" :: "m"(*addr) : "memory" );
-            }
+
+            auto *addr = reinterpret_cast<const uint8_t *>(it.vaddress());
+            asm ( "invlpg %0" :: "m"(*addr) : "memory" );
+
             if (free_count && phys == free_start + (free_count * frame_size)) {
                 ++free_count;
             } else {