riscv: turn mm_segment_t into a struct

This matches what other heavily used architectures do, and will allow us
to easily use <asm-generic/uaccess.h> for the nommu case.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>

arch/riscv/include/asm/thread_info.h

@@ -28,7 +28,9 @@
 #include <asm/processor.h>
 #include <asm/csr.h>
 
-typedef unsigned long mm_segment_t;
+typedef struct {
+	unsigned long seg;
+} mm_segment_t;
 
 /*
  * low level task data that entry.S needs immediate access to