signal/arm64: Only call set_thread_esr once in do_page_fault

This code is truly common between the signal sending cases so share it.

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>