x86/sgx: Fix missing poison handling in reclaimer

The SGX reclaimer code lacks page poison handling in its main
free path. This can lead to avoidable machine checks if a
poisoned page is freed and reallocated instead of being
isolated.

A troublesome scenario is:
 1. Machine check (#MC) occurs (asynchronous, !MF_ACTION_REQUIRED)
 2. arch_memory_failure() is eventually called
 3. (SGX) page->poison set to 1
 4. Page is reclaimed
 5. Page added to normal free lists by sgx_reclaim_pages()
    ^ This is the bug (poison pages should be isolated on the
    sgx_poison_page_list instead)
 6. Page is reallocated by some innocent enclave, a second (synchronous)
    in-kernel #MC is induced, probably during EADD instruction.
    ^ This is the fallout from the bug

(6) is unfortunate and can be avoided by replacing the open coded
enclave page freeing code in the reclaimer with sgx_free_epc_page()
to obtain support for poison page handling that includes placing the
poisoned page on the correct list.

Fixes: d6d261bd ("x86/sgx: Add new sgx_epc_page flag bit to mark free pages")
Fixes: 992801ae ("x86/sgx: Initial poison handling for dirty and free pages")
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Link: https://lkml.kernel.org/r/dcc95eb2aaefb042527ac50d0a50738c7c160dac.1643830353.git.reinette.chatre@intel.com