MIPS: cacheflush: Use __flush_icache_user_range()

The cacheflush(2) system call uses flush_icache_range() to flush a range
of usermode addresses from the icache, so change it to utilise the new
__flush_icache_user_range() API to allow the more generic
flush_icache_range() to be changed to work on kernel addresses only.

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/14153/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>