Make it possible to disable the kuser helpers by adding a KUSER_HELPERS config option (enabled by default). When disabled, all kuser helpers-related code is removed from the kernel and no mapping is done at the fixed high address (0xffff0000); any attempt to use a kuser helper from a 32-bit process will result in a segfault.
Signed-off-by: Mark Salyzyn <[email protected]> v2: - vectors_page and compat_vdso_spec as array of 2 - free sigpage if vectors allocation failed - adjust makefile so one line for each of the assembler source modules - split off assembler changes to a new previous patch in series to reduce churn - modify slightly the feature documentation to reduce its reach - modify slightly the feature documentation to rationalize the yes default. - There are more ifdefs as a result of the rebase. v3: - rebase (minor conflicts)
