mm/mprotect.c: avoid repeated assignment in do_mprotect_pkey()

After adjustment, the repeated assignment of "prev" is avoided, and the
readability of the code is improved.

Link: https://lkml.kernel.org/r/20211012152444.4127-1-fishland@aliyun.com
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Liu Song <liu.song11@zte.com.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mprotect.c

@@ -563,7 +563,7 @@ static int do_mprotect_pkey(unsigned long start, size_t len,
 	error = -ENOMEM;
 	if (!vma)
 		goto out;
-	prev = vma->vm_prev;
+
 	if (unlikely(grows & PROT_GROWSDOWN)) {
 		if (vma->vm_start >= end)
 			goto out;
@@ -581,8 +581,11 @@ static int do_mprotect_pkey(unsigned long start, size_t len,
 				goto out;
 		}
 	}
+
 	if (start > vma->vm_start)
 		prev = vma;
+	else
+		prev = vma->vm_prev;
 
 	for (nstart = start ; ; ) {
 		unsigned long mask_off_old_flags;