x86: MMU: Bug in _x86_mmu_buffer_validate

The value of the PTE (starting_pte_num) was not
calulated correctly. If size of the buffer exceeded 4KB,
the buffer validation API was failing.

JIRA: ZEP-2489

Signed-off-by: Adithya Baglody <adithya.nagaraj.baglody@intel.com>