make do_mremap() static

The extern in sys_sparc_64.c was a rudiment of time when do_mremap()
used to exist in MMU case (it doesn't anymore).  As for !MMU one,
nothing uses it outside of mm/nommu.c...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/mm/nommu.c b/mm/nommu.c
index e193280..66737e0 100644
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -1770,7 +1770,7 @@
  *
  * MREMAP_FIXED is not supported under NOMMU conditions
  */
-unsigned long do_mremap(unsigned long addr,
+static unsigned long do_mremap(unsigned long addr,
 			unsigned long old_len, unsigned long new_len,
 			unsigned long flags, unsigned long new_addr)
 {
@@ -1805,7 +1805,6 @@
 	vma->vm_end = vma->vm_start + new_len;
 	return vma->vm_start;
 }
-EXPORT_SYMBOL(do_mremap);
 
 SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
 		unsigned long, new_len, unsigned long, flags,