arm64/fpsimd: signal: Consistently read FPSIMD context

For historical reasons, restore_sve_fpsimd_context() has an open-coded
copy of the logic from read_fpsimd_context(), which is used to either
restore an FPSIMD-only context, or to merge FPSIMD state into an
SVE state when restoring an SVE+FPSIMD context. The logic is *almost*
identical.

Refactor the logic to avoid duplication and make this clearer.

This comes with two functional changes that I do not believe will be
problematic in practice:

* The user_fpsimd_state::size field will be checked in all restore paths
  that consume it user_fpsimd_state. The kernel always populates this
  field when delivering a signal, and so this should contain the
  expected value unless it has been corrupted.

* If a read of user_fpsimd_state fails, we will return early without
  modifying TIF_SVE, the saved SVCR, or the save fp_type. This will
  leave the task in a consistent state, without potentially resurrecting
  stale FPSIMD state. A read of user_fpsimd_state should never fail
  unless the structure has been corrupted or the stack has been
  unmapped.

Suggested-by: Will Deacon <will@kernel.org>
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/20250508132644.1395904-5-mark.rutland@arm.com


[will: Ensure read_fpsimd_context() returns negative error code or zero]
Signed-off-by: Will Deacon <will@kernel.org>