x86/efi: Use efi_switch_mm() rather than manually twiddling with %cr3

Use helper function efi_switch_mm() to switch to/from efi_mm when
invoking any UEFI runtime services.

Likewise, we need to switch back to previous mm (mm context stolen
by efi_mm) after the above calls return successfully. We can use
efi_switch_mm() helper function only with x86_64 kernel and
"efi=old_map" disabled because, x86_32 and efi=old_map do not use
efi_pgd, rather they use swapper_pg_dir.

Tested-by: Bhupesh Sharma <bhsharma@redhat.com>
[ardb: add #include of sched/task.h for task_lock/_unlock]
Signed-off-by: Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Matt Fleming <matt@codeblueprint.co.uk>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Lee, Chun-Yi <jlee@suse.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ravi Shankar <ravi.v.shankar@intel.com>
Cc: Ricardo Neri <ricardo.neri@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tony Luck <tony.luck@intel.com>
Cc: linux-efi@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>