The protected_writeback_scache_line() function is used by
local_r4k_flush_cache_sigtramp() to flush an FPU delay slot emulation
trampoline on the userland stack from the caches so it is visible to
subsequent instruction fetches.

Commit de8974e3 ("MIPS: asm: r4kcache: Add EVA cache flushing
functions") updated some protected_ cache flush functions to use EVA
CACHEE instructions via protected_cachee_op(), and commit 83fd4344
("MIPS: r4kcache: Add EVA case for protected_writeback_dcache_line") did
the same thing for protected_writeback_dcache_line(), but
protected_writeback_scache_line() never got updated. Lets fix that now
to flush the right user address from the secondary cache rather than
some arbitrary kernel unmapped address.

This issue was spotted through code inspection, and it seems unlikely to
be possible to hit this in practice. It theoretically affect EVA kernels
on EVA capable cores with an L2 cache, where the icache fetches straight
from RAM (cpu_icache_snoops_remote_store == 0), running a hard float
userland with FPU disabled (nofpu). That both Malta and Boston platforms
override cpu_icache_snoops_remote_store to 1 suggests that all MIPS
cores fetch instructions into icache straight from L2 rather than RAM.

Fixes: de8974e3 ("MIPS: asm: r4kcache: Add EVA cache flushing functions")
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Leonid Yegoshin <leonid.yegoshin@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/13800/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>