kernel: Reinstate ZERO_PAGE optimization in 'get_user_pages()' and fix XIP

The Linux kernel 2.6.24 and 2.6.25 before 2.6.25.9 allows local users to cause a denial of service memory consumption via a large number of calls to the getuserpages function, which lacks a ZEROPAGE optimization and results in allocation of "useless newly zeroed pages."...