x86/speculation: Use cached host SPEC_CTRL value for guest entry/exit

commit bbb69e8b upstream.

There's no need to recalculate the host value for every entry/exit.
Just use the cached value in spec_ctrl_current().

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>