`PageTableFlags::PAT_HUGE_PAGE` overlaps with address field

[phil-opp]

The PAT_HUGE_PAGE is bit 1 << 12, which is usually part of the address field. So we mask it out:

x86_64/src/structures/paging/page_table.rs

Lines 109 to 111 in 286ea1f

	const fn physical_address_mask() -> u64 {
	0x000f_ffff_ffff_f000
	}

This mask is used in many places. This means that PAT_HUGE_PAGE will be never returned by flags even if it is set:

x86_64/src/structures/paging/page_table.rs

Lines 57 to 59 in 286ea1f

	pub const fn flags(&self) -> PageTableFlags {
	PageTableFlags::from_bits_retain(self.entry & !Self::physical_address_mask())
	}

Another issue is that set_flags/set_addr will blindly OR PAT_HUGE_PAGE in, but this will change the address when applied to a non-huge entry:

x86_64/src/structures/paging/page_table.rs

Lines 103 to 105 in 286ea1f

	pub fn set_flags(&mut self, flags: PageTableFlags) {
	self.entry = self.addr().as_u64() | flags.bits();
	}

Given that the PAT_HUGE_PAGE only lives on next (introduced in #548), it's probably a good idea to remove it again before cutting the 0.16 release (see #600). We could replace it with a getter/setter (needs to take is_level_1_table as arg).