atapi: Account for failed and invalid operations in cd_read_sector()

Commit 5f81724d made PIO read requests async but didn't add the
relevant block_acct_failed() and block_acct_invalid() calls.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-id: 9b87e09d61019c128139b6c999ed0c07f0674170.1448367341.git.berto@igalia.com
Signed-off-by: John Snow <jsnow@redhat.com>
