Caught during Trinity testing. The pte_modify does not allow
modification for PTE type bit. This cause the test to hang
the system. It is found that the PTE can't transit from an
inaccessible page (b00) to a valid page (b11) because the mask
does not allow it. This happens when a big block of mmaped
memory is set the PROT_NONE, then the a small piece is broken
off and set to PROT_WRITE | PROT_READ cause a huge page split.

Signed-off-by: Feng Kan <fkan@apm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>