Fix for the vsyscall segment & support for the uffd-wp bit

[tramasys]

Heya, just shooting you a tiny commit with an addition and a fix for the vsyscall segment.

uffd-wp

This just extends the PageMapEntry impl, to include the 57th bit for the uffd-wp flag (available since 5.13, see: https://www.kernel.org/doc/Documentation/admin-guide/mm/userfaultfd.rst)

vsyscall

Using the start address of the vsyscall segment to index into the pagemap will always return an empty buffer (i.e for a pread64 syscall). This causes an PageMapError::Read in your code in the pagemap_vma function (it's an UnexpectedEof to be precise). By filtering out this segment in the pagemap function, we can prevent this error from happening.