Kernel: Read the ELF header from the inode rather than the mapped pages

Reading from the mapping doesn't work when the text segment has a non-zero
offset because in that case the first mapped page doesn't contain the ELF
header.

Kernel/Syscalls/mmap.cpp

@@ -64,12 +64,15 @@ static bool should_make_executable_exception_for_dynamic_loader(bool make_readab
  if (!region.vmobject().is_private_inode())
  return false;
 
+ auto& inode_vm = static_cast<const InodeVMObject&>(region.vmobject());
+ auto& inode = inode_vm.inode();
+
  Elf32_Ehdr header;
- if (!copy_from_user(&header, region.vaddr().as_ptr(), sizeof(header)))
+ auto buffer = UserOrKernelBuffer::for_kernel_buffer((u8*)&header);
+ auto nread = inode.read_bytes(0, sizeof(header), buffer, nullptr);
+ if (nread != sizeof(header))
  return false;
 
- auto& inode = static_cast<const InodeVMObject&>(region.vmobject());
-
  // The file is a valid ELF binary
  if (!ELF::validate_elf_header(header, inode.size()))
  return false;