ecryptfs: reject oversized encrypted_key_size in parse_tag_3_packet

In the Linux kernel, the following vulnerability has been resolved:

ecryptfs: reject oversized encrypted_key_size in parse_tag_3_packet

parse_tag_3_packet() set encrypted_key_size from the Tag 3 packet body
without bounding it against ECRYPTFS_MAX_KEY_BYTES (64). When
encrypted_key_size > 64, decrypt_passphrase_encrypted_session_key()
sets decrypted_key_size = encrypted_key_size and performs two
out-of-bounds writes:

1. crypto_skcipher_decrypt() writes encrypted_key_size bytes into
decrypted_key[64] via scatterlist, overflowing into the parent
ecryptfs_auth_tok struct.
2. memcpy(crypt_stat->key, decrypted_key, decrypted_key_size) writes
into crypt_stat->key[64], corrupting root_iv, keysig_list, and
mutexes in ecryptfs_crypt_stat.

Only AES-192 (cipher code 0x08) enables this because it sets
crypt_stat->key_size = 24 independently of encrypted_key_size,
allowing crypto_skcipher_setkey() to succeed while encrypted_key_size
exceeds ECRYPTFS_MAX_KEY_BYTES.

The PKI decryption path (parse_tag_65_packet) already validates
decrypted_key_size <= ECRYPTFS_MAX_KEY_BYTES; the passphrase path
omits this check.

Bound encrypted_key_size against ECRYPTFS_MAX_KEY_BYTES (64) rather
than ECRYPTFS_MAX_ENCRYPTED_KEY_BYTES (512). The 64-byte limit also
protects the 512-byte encrypted_key[] buffer, so the former 512-byte
check is removed as redundant.

[tyhicks: Adjust the code comment to refer to macros representing the
buffer sizes rather than mentioning the buffer size values since they
may change in the future]