selftests/sgx: Rename 'eenter' and 'sgx_call_vdso'

Rename symbols for better clarity:

* 'eenter' might be confused for directly calling ENCLU[EENTER].  It does
  not.  It calls into the VDSO, which actually has the EENTER instruction.
* 'sgx_call_vdso' is *only* used for entering the enclave.  It's not some
  generic SGX call into the VDSO.

Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>