x86: fix incorrect __read_mostly on _boot_cpu_pda

The pda rework (commit 3461b0af025251bbc6b3d56c821c6ac2de6f7209)
to remove static boot cpu pdas introduced a performance bug.

_boot_cpu_pda is the actual pda used by the boot cpu and is definitely
not "__read_mostly" and ended up polluting the read mostly section with
writes. This bug caused regression of about 8-10% on certain syscall
intensive workloads.

Signed-off-by: Ravikiran Thirumalai <[e-mail]>
Acked-by: Mike Travis <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>

arch/x86/kernel/head64.c

···
 #include <asm/trampoline.h>
 
 /* boot cpu pda */
-static struct x8664_pda _boot_cpu_pda __read_mostly;
+static struct x8664_pda _boot_cpu_pda;
 
 #ifdef CONFIG_SMP
 /*