x86: CPA fix pagetable split

Move the readout of the large entry into the spinlock section to
prevent an unlikely but possible race.

Mark the pmd/pud entry present after the split. We preserved the
non present bit in the new split mapping.

Remove the stale gfp_flags double initialization.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>