x86/sgx: Add SGX_CHILD_PRESENT hardware error code

SGX driver can accurately track how enclave pages are used.  This
enables SECS to be specifically targeted and EREMOVE'd only after all
child pages have been EREMOVE'd.  This ensures that SGX driver will
never encounter SGX_CHILD_PRESENT in normal operation.

Virtual EPC is different.  The host does not track how EPC pages are
used by the guest, so it cannot guarantee EREMOVE success.  It might,
for instance, encounter a SECS with a non-zero child count.

Add a definition of SGX_CHILD_PRESENT.  It will be used exclusively by
the SGX virtualization driver to handle recoverable EREMOVE errors when
saniziting EPC pages after they are freed.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Kai Huang <kai.huang@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Dave Hansen <dave.hansen@intel.com>
Acked-by: Jarkko Sakkinen <jarkko@kernel.org>
Link: https://lkml.kernel.org/r/050b198e882afde7e6eba8e6a0d4da39161dbb5a.1616136308.git.kai.huang@intel.com

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