arm64/fpsimd: Remove redundant task->mm check

arm64/fpsimd: Remove redundant task->mm check

For historical reasons, arch_dup_task_struct() only calls
fpsimd_preserve_current_state() when current->mm is non-NULL, but this
is no longer necessary.

Historically TIF_FOREIGN_FPSTATE was only managed for user threads, and
was never set for kernel threads. At the time, various functions
attempted to avoid saving/restoring state for kernel threads by checking
task_struct::mm to try to distinguish user threads from kernel threads.

We added the current->mm check to arch_dup_task_struct() in commit:

  6eb6c80187c5 ("arm64: kernel thread don't need to save fpsimd context.")

... where the intent was to avoid pointlessly saving state for kernel
threads, which never had live state (and the saved state would never be
consumed).

Subsequently we began setting TIF_FOREIGN_FPSTATE for kernel threads,
and removed most of the task_struct::mm checks in commit:

  df3fb9682045 ("arm64: fpsimd: Eliminate task->mm checks")

... but we missed the check in arch_dup_task_struct(), which is similarly
redundant.

Remove the redundant check from arch_dup_task_struct().

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20250508132644.1395904-11-mark.rutland@arm.com
Signed-off-by: Will Deacon <will@kernel.org>