efi_loader: correct reading of directories

efi_loader: correct reading of directories

EFI_FILE_PROTOCOL.Read() is used both to read files and directories.

When reaching the end of a directory we always have to return buffer size
zero irrespective of the incoming buffer size. (The described scenario for
a Shim quirk cannot arise because every directory has at least '.' and '..'
as entries.)

Even when the buffer_size is too small multiple times we have to keep a
reference to our last read directory entry.

When we return to the start of the directory via SetPosition() we must
remove the reference to a previously kept directory entry.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>