Fix handling of bad file descriptors in dup(2).

Previously, sys_dup() would do dtable->Get() on the passed in-file
descriptor and then pass the result directly to dtable->Allocate(). If
the file descriptor is not valid, dtable->Get() returns a NULL reference
and sets errno to mark the error. Since sys_dup() did not check the
return value of dtable->Get() and dtable->Allocate() does not check
whether the passed in Ref<Descriptor> is a NULL reference, dup(2) with
invalid file descriptor would succesfully allocate a new file descriptor
with garbage contents.

This commit changes sys_dup() to use a variant of dtable->Allocate()
that takes in a file descriptor as an integer and properly validates it
before use.

kernel/io.cpp

@@ -138,9 +138,7 @@ int sys_closefrom(int fd)
 
 int sys_dup(int fd)
 {
-	Ref<DescriptorTable> dtable = CurrentProcess()->GetDTable();
-	Ref<Descriptor> desc = dtable->Get(fd);
-	return dtable->Allocate(desc, 0);
+	return CurrentProcess()->GetDTable()->Allocate(fd, 0);
 }
 
 int sys_dup3(int oldfd, int newfd, int flags)