gpg: Accept Ed25519 private key in SOS which reserves leading zeros.
* g10/parse-packet.c (sos_read): Backport from 2.3. (parse_key): Use sos_read for Ed25519 private key.
Note that we keep the code of sos_read as same as 2.3. Even it is
set, the GCRYMPI_FLAG_USER2 flag is not used.
- GnuPG-bug-id: T5120
- Signed-off-by: NIIBE Yutaka <email@example.com>