f2fs: use rw_semaphore to protect SIT cache

There are some cases user didn't update SIT cache under this lock,
so let's use rw_semaphore instead of mutex to enhance concurrently
accessing.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fs/f2fs/segment.h b/fs/f2fs/segment.h
index 8d93652..245acb3 100644
--- a/fs/f2fs/segment.h
+++ b/fs/f2fs/segment.h
@@ -231,7 +231,7 @@
 	unsigned long *dirty_sentries_bitmap;	/* bitmap for dirty sentries */
 	unsigned int dirty_sentries;		/* # of dirty sentries */
 	unsigned int sents_per_block;		/* # of SIT entries per block */
-	struct mutex sentry_lock;		/* to protect SIT cache */
+	struct rw_semaphore sentry_lock;	/* to protect SIT cache */
 	struct seg_entry *sentries;		/* SIT segment-level cache */
 	struct sec_entry *sec_entries;		/* SIT section-level cache */