arm64: Rewrite __arch_clear_user()

arm64: Rewrite __arch_clear_user()

Now that we're always using STTR variants rather than abstracting two
different addressing modes, the user_ldst macro here is frankly more
obfuscating than helpful. Rewrite __arch_clear_user() with regular
USER() annotations so that it's clearer what's going on, and take the
opportunity to minimise the branchiness in the most common paths, while
also allowing the exception fixup to return an accurate result.

Apparently some folks examine large reads from /dev/zero closely enough
to notice the loop being hot, so align it per the other critical loops
(presumably around a typical instruction fetch granularity).

Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/1cbd78b12c076a8ad4656a345811cfb9425df0b3.1622128527.git.robin.murphy@arm.com
Signed-off-by: Will Deacon <will@kernel.org>