arm64: fpsimd: Drop unneeded 'busy' flag

Kernel mode NEON will preserve the user mode FPSIMD state by saving it
into the task struct before clobbering the registers. In order to avoid
the need for preserving kernel mode state too, we disallow nested use of
kernel mode NEON, i..e, use in softirq context while the interrupted
task context was using kernel mode NEON too.

Originally, this policy was implemented using a per-CPU flag which was
exposed via may_use_simd(), requiring the users of the kernel mode NEON
to deal with the possibility that it might return false, and having NEON
and non-NEON code paths. This policy was changed by commit
13150149

 ("arm64: fpsimd: run kernel mode NEON with softirqs
disabled"), and now, softirq processing is disabled entirely instead,
and so may_use_simd() can never fail when called from task or softirq
context.

This means we can drop the fpsimd_context_busy flag entirely, and
instead, ensure that we disable softirq processing in places where we
formerly relied on the flag for preventing races in the FPSIMD preserve
routines.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20231208113218.3001940-7-ardb@google.com
[will: Folded in fix from CAMj1kXFhzbJRyWHELCivQW1yJaF=p07LLtbuyXYX3G1WtsdyQg@mail.gmail.com]
Signed-off-by: Will Deacon <will@kernel.org>