arm64/kpti: Move DAIF masking to C code

arm64/kpti: Move DAIF masking to C code

We really don't want to take an exception while replacing TTBR1 so we mask
DAIF during the actual update. Currently this is done in the assembly
function idmap_cpu_replace_ttbr1() but it could equally be done in the only
caller of that function, cpu_replace_ttbr1(). This simplifies the assembly
code slightly and means that when working with the code around masking DAIF
flags there is one less piece of assembly code which needs to be considered.

While we're at it add a comment which makes explicit why we are masking
DAIF in this code.

There should be no functional effect.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20221123180209.634650-2-broonie@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>