signal/arm64: For clarity separate the 3 signal sending cases in do_page_fault

It gets easy to confuse what is going on when some code is shared and some not
so stop sharing the trivial bits of signal generation to make future updates
easier to understand.

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>