x86/fpu: Extract restore_from_ia32_fxstate() and clean up fpu__restore_sig()

When restoring an FPU signal frame for a 32-bit or IA32-compat task on
FXSR-enabled systems, a legacy 32-bit FP frame is present alongside the
FX/XSAVE frame. Because the legacy FP frame duplicates the FP state
portion of the FX/XSAVE frame, for backward compatibility it is treated
as the source of truth, and its state is folded into the FX/XSAVE state
before restoring the registers.

Currently, most of __fpu_restore_sig() is dedicated to handling this
32-bit legacy/compat fpstate, while the native direct restore path lives
in restore_fpregs_from_user(). Having the compat handling intermixed
with the main signal restoration flow makes it tricky to quickly see
what code is doing what.

Extract the 32-bit legacy/compat FPU restore handling into a separate
helper function, restore_from_ia32_fxstate(), and inline the remainder
of __fpu_restore_sig() directly into fpu__restore_sig().

Signed-off-by: Andrei Vagin <avagin@google.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Alexander Mikhalitsyn <alexander@mihalicyn.com>
Reviewed-by: Chang S. Bae <chang.seok.bae@intel.com>
Link: https://patch.msgid.link/20260925162454.1403405-4-avagin@google.com

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