arm64: uaccess: Cleanup get/put_user()

__get/put_user_check() macro is made to return a value but this is never
used. Get rid of them and just use directly __get/put_user_error() as
a statement, reducing macro indirection.

Also, take this opportunity to rename __get/put_user_err() as it gets
a bit confusing having them along __get/put_user_error().

Signed-off-by: Julien Thierry <julien.thierry@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>