loader/efi/linux: Return correct size from LoadFile2

loader/efi/linux: Return correct size from LoadFile2

From UEFI specifications 2.10, section 13.2.2, EFI_LOAD_FILE2_PROTOCOL.LoadFile
(see https://uefi.org/specs/UEFI/2.10/13_Protocols_Media_Access.html), for BufferSize:

  On input the size of Buffer in bytes. On output with a return code
  of EFI_SUCCESS, the amount of data transferred to Buffer. On output
  with a return code of EFI_BUFFER_TOO_SMALL, the size of Buffer
  required to retrieve the requested file.

So, set *buffer_size correctly.

Signed-off-by: Frediano Ziglio <frediano.ziglio@cloud.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>