x86/sgx: Do not fail on incomplete sanitization on premature stop of ksgxd

Unsanitized pages trigger WARN_ON() unconditionally, which can panic the
whole computer, if /proc/sys/kernel/panic_on_warn is set.

In sgx_init(), if misc_register() fails or misc_register() succeeds but
neither sgx_drv_init() nor sgx_vepc_init() succeeds, then ksgxd will be
prematurely stopped. This may leave unsanitized pages, which will result a
false warning.

Refine __sgx_sanitize_pages() to return:

1. Zero when the sanitization process is complete or ksgxd has been
   requested to stop.
2. The number of unsanitized pages otherwise.

Fixes: 51ab30eb2ad4 ("x86/sgx: Replace section->init_laundry_list with sgx_dirty_page_list")
Reported-by: Paul Menzel <[email protected]>
Signed-off-by: Jarkko Sakkinen <[email protected]>
Signed-off-by: Dave Hansen <[email protected]>
Reviewed-by: Reinette Chatre <[email protected]>
Cc: [email protected]
Link: https://lore.kernel.org/linux-sgx/[email protected]/T/#u
Link: https://lkml.kernel.org/r/[email protected]

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