FROMLIST: [PATCH v2 3/3] arm64: compat: Add CONFIG_KUSER_HELPERS

FROMLIST: [PATCH v2 3/3] arm64: compat: Add CONFIG_KUSER_HELPERS

(cherry picked from url http://lkml.iu.edu/hypermail/linux/kernel/1709.1/01903.html)

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: Kevin Brodsky <kevin.brodsky@arm.com>
Signed-off-by: Mark Salyzyn <salyzyn@android.com>
Bug: 9674955
Bug: 63737556
Bug: 20045882
Change-Id: Ie8c543301d39bfe88ef71fb6a669e571914b117b