Kernel: Make file-backed memory regions remember description permissions

This allows sys$mprotect() to honor the original readable & writable
flags of the open file description as they were at the point we did the
original sys$mmap().

IIUC, this is what Dr. POSIX wants us to do:
https://pubs.opengroup.org/onlinepubs/9699919799/functions/mprotect.html

Also, remove the bogus and racy "W^X" checking we did against mappings
based on their current inode metadata. If we want to do this, we can do
it properly. For now, it was not only racy, but also did blocking I/O
while holding a spinlock.

Kernel/Memory/InodeVMObject.cpp

@@ -96,14 +96,4 @@ u32 InodeVMObject::writable_mappings() const
     return count;
 }
 
-u32 InodeVMObject::executable_mappings() const
-{
-    u32 count = 0;
-    const_cast<InodeVMObject&>(*this).for_each_region([&](auto& region) {
-        if (region.is_executable())
-            ++count;
-    });
-    return count;
-}
-
 }