Add a wrapper for the EMODPR ENCLS leaf function used to
restrict enclave page permissions as maintained in the
SGX hardware's Enclave Page Cache Map (EPCM).

EMODPR:
1) Updates the EPCM permissions of an enclave page by treating
   the new permissions as a mask. Supplying a value that attempts
   to relax EPCM permissions has no effect on EPCM permissions
   (PR bit, see below, is changed).
2) Sets the PR bit in the EPCM entry of the enclave page to
   indicate that permission restriction is in progress. The bit
   is reset by the enclave by invoking ENCLU leaf function
   EACCEPT or EACCEPTCOPY.

The enclave may access the page throughout the entire process
if conforming to the EPCM permissions for the enclave page.

After performing the permission restriction by issuing EMODPR
the kernel needs to collaborate with the hardware to ensure that
all logical processors sees the new restricted permissions. This
is required for the enclave's EACCEPT/EACCEPTCOPY to succeed and
is accomplished with the ETRACK flow.

Expand enum sgx_return_code with the possible EMODPR return
values.
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/d15e7a769e13e4ca671fa2d0a0d3e3aec5aedbd4.1652137848.git.reinette.chatre@intel.com