x86/paravirt: Make read_cr2() CALLEE_SAVE

The one paravirt read_cr2() implementation (Xen) is actually quite trivial
and doesn't need to clobber anything other than the return register.

Making read_cr2() CALLEE_SAVE avoids all the PUSH/POP nonsense and allows
more convenient use from assembly.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Juergen Gross <jgross@suse.com>
Cc: bp@alien8.de
Cc: rostedt@goodmis.org
Cc: luto@kernel.org
Cc: torvalds@linux-foundation.org
Cc: hpa@zytor.com
Cc: dave.hansen@linux.intel.com
Cc: zhe.he@windriver.com
Cc: joel@joelfernandes.org
Cc: devel@etsukata.com
Link: https://lkml.kernel.org/r/20190711114335.887392493@infradead.org