m68k: Remove set_fs()

Add a m68k-only set_fc helper to set the SFC and DFC registers for the
few places that need to override it for special MM operations, but
disconnect that from the deprecated kernel-wide set_fs() API.

Note that the SFC/DFC registers are context switched, so there is no need
to disable preemption.

Partially based on an earlier patch from
Linus Torvalds <torvalds@linux-foundation.org>.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Michael Schmitz <schmitzmic@gmail.com>
Tested-by: Michael Schmitz <schmitzmic@gmail.com>
Link: https://lore.kernel.org/r/20210916070405.52750-7-hch@lst.de
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>

arch/m68k/include/asm/uaccess.h

@@ -9,7 +9,6 @@
  */
 #include <linux/compiler.h>
 #include <linux/types.h>
-#include <asm/segment.h>
 #include <asm/extable.h>
 
 /* We let the MMU do all checking */
@@ -451,9 +450,6 @@ do {									\
 		goto err_label;						\
 } while (0)
 
-#define user_addr_max() \
-	(uaccess_kernel() ? ~0UL : TASK_SIZE)
-
 extern long strncpy_from_user(char *dst, const char __user *src, long count);
 extern __must_check long strnlen_user(const char __user *str, long n);