1) Fix a potential overflow in user_pcid_flush_mask.
   I do not think anybody is hitting this in practice,
   but they could if they wanted to.

2) Change the early boot initialized value of invlpgb_count_max
   to 1, to avoid an infinite loop when...

3) Having cpa_flush() call flush_kernel_range(), which results
   in the INVPLGB code being called very early at boot time.

v2:
- changelog improvements (Dave Hansen)
- get rid of #ifdefs in .c files (Dave Hansen)