x86/sgx: Export sgx_encl_ewb_cpumask()

Using sgx_encl_ewb_cpumask() to learn which CPUs might have executed
an enclave is useful to ensure that TLBs are cleared when changes are
made to enclave pages.

sgx_encl_ewb_cpumask() is used within the reclaimer when an enclave
page is evicted. The upcoming SGX2 support enables changes to be
made to enclave pages and will require TLBs to not refer to the
changed pages and thus will be needing sgx_encl_ewb_cpumask().

Relocate sgx_encl_ewb_cpumask() to be with the rest of the enclave
code in encl.c now that it is no longer unique to the reclaimer.

Take care to ensure that any future usage maintains the
current context requirement that ETRACK has been called first.
Expand the existing comments to highlight this while moving them
to a more prominent location before the function.

No functional change.

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/05b60747fd45130cf9fc6edb1c373a69a18a22c5.1652137848.git.reinette.chatre@intel.com