efi_loader: avoid pointer access after calling efi_delete_handle

efi_delete_handle() calls efi_purge_handle(), then it finally
frees the EFI handle.
Both diskobj and handle variables in efi_disk_remove() have
the same pointer, we can not access diskobj->dp after calling
efi_delete_handle().

This commit saves the struct efi_device_path pointer before
calling efi_delete_handle(). This commit also fixes the
missing free for volume member in struct efi_disk_obj.

This commit also removes the container_of() calls, and
adds the TODO comment of missing efi_close_protocol() call
for the parent EFI_BLOCK_IO_PROTOCOL.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>