x86/sgx: Add hook to error injection address validation

SGX reserved memory does not appear in the standard address maps.

Add hook to call into the SGX code to check if an address is located
in SGX memory.

There are other challenges in injecting errors into SGX. Update the
documentation with a sequence of operations to inject.

Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Tested-by: Reinette Chatre <reinette.chatre@intel.com>
Link: https://lkml.kernel.org/r/20211026220050.697075-7-tony.luck@intel.com