MIPS: Fix a preemption issue with thread's FPU defaults

MIPS: Fix a preemption issue with thread's FPU defaults

Fix "BUG: using smp_processor_id() in preemptible" reported in accesses
to thread's FPU defaults: the value to initialise FSCR to at program
startup, the FCSR r/w mask and the contents of FIR in full FPU
emulation, removing a regression introduced with 9b26616c [MIPS: Respect
the ISA level in FCSR handling] and f6843626 [MIPS: math-emu: Set FIR
feature flags for full emulation].

Use `boot_cpu_data' to obtain the data from, following the approach that
`cpu_has_*' macros take and avoiding the call to `smp_processor_id' made
in the reference to `current_cpu_data'.  The contents of FSCR have to be
consistent across processors in an SMP system, the settings there must
not change as a thread is migrated across processors.  And the contents
of FIR are guaranteed to be consistent in FPU emulation, by definition.

Signed-off-by: Maciej W. Rozycki <[email protected]>
Tested-by: Ezequiel Garcia <[email protected]>
Tested-by: Paul Martin <[email protected]>
Cc: Markos Chandras <[email protected]>
Cc: James Hogan <[email protected]>
Cc: [email protected]
Patchwork: https://patchwork.linux-mips.org/patch/10030/
Signed-off-by: Ralf Baechle <[email protected]>