mm: nommu: correct the range of mmap_sem_read_lock in task_mem()

During the seq_printf,the mmap_sem_read_lock protection is not
required.

Link: https://lkml.kernel.org/r/20230622040152.1173-1-lipeifeng@oppo.com
Signed-off-by: lipeifeng <lipeifeng@oppo.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: lipeifeng-china

fs/proc/task_nommu.c

@@ -51,7 +51,7 @@ void task_mem(struct seq_file *m, struct mm_struct *mm)
 		sbytes += kobjsize(mm);
 	else
 		bytes += kobjsize(mm);
-	
+
 	if (current->fs && current->fs->users > 1)
 		sbytes += kobjsize(current->fs);
 	else
@@ -69,13 +69,13 @@ void task_mem(struct seq_file *m, struct mm_struct *mm)
 
 	bytes += kobjsize(current); /* includes kernel stack */
 
+	mmap_read_unlock(mm);
+
 	seq_printf(m,
 		"Mem:\t%8lu bytes\n"
 		"Slack:\t%8lu bytes\n"
 		"Shared:\t%8lu bytes\n",
 		bytes, slack, sbytes);
-
-	mmap_read_unlock(mm);
 }
 
 unsigned long task_vsize(struct mm_struct *mm)