signal/arm64: Factor set_thread_esr out of __do_user_fault

This pepares for sending signals with something other than
arm64_force_sig_info.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Tested-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>