x86/sgx: Replace kmap/kunmap_atomic() calls

kmap_local_page() is the preferred way to create temporary mappings when it
is feasible, because the mappings are thread-local and CPU-local.

kmap_local_page() uses per-task maps rather than per-CPU maps. This in
effect removes the need to disable preemption on the local CPU while the
mapping is active, and thus vastly reduces overall system latency. It is
also valid to take pagefaults within the mapped region.

The use of kmap_atomic() in the SGX code was not an explicit design choice
to disable page faults or preemption, and there is no compelling design
reason to using kmap_atomic() vs. kmap_local_page().

Signed-off-by: Kristen Carlson Accardi <kristen@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Reviewed-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Link: https://lore.kernel.org/linux-sgx/Y0biN3%2FJsZMa0yUr@kernel.org/
Link: https://lore.kernel.org/r/20221115161627.4169428-1-kristen@linux.intel.com