arm64/fpsimd: Allocate kernel mode FP/SIMD buffers on the stack

Commit aefbab8e77eb16b5

  ("arm64: fpsimd: Preserve/restore kernel mode NEON at context switch")

added a 'kernel_fpsimd_state' field to struct thread_struct, which is
the arch-specific portion of struct task_struct, and is allocated for
each task in the system. The size of this field is 528 bytes, resulting
in non-trivial bloat of task_struct, and the resulting memory overhead
may impact performance on systems with many processes.

This allocation is only used if the task is scheduled out or interrupted
by a softirq while using the FP/SIMD unit in kernel mode, and given that
calls to kernel_neon_begin() and kernel_neon_end() are now guaranteed to
originate from the same lexical scope, it is possible to transparently
allocate this buffer on the caller's stack instead.

So tweak the 'ksimd' scoped guard implementation so that a stack buffer
is allocated and passed to both kernel_neon_begin() and
kernel_neon_end(), and record it in the task struct. Passing the address
to both functions, and checking the addresses for consistency ensures
that callers of the updated bare begin/end API use it in a manner that
is consistent with the new context switch semantics.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>