f2fs: add a tracepoint for f2fs_write_begin

This patch adds a tracepoint for f2fs_write_begin to trace write op of user.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>
diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
index a7bb98f..cde0d69 100644
--- a/fs/f2fs/data.c
+++ b/fs/f2fs/data.c
@@ -900,6 +900,8 @@
 	struct dnode_of_data dn;
 	int err = 0;
 
+	trace_f2fs_write_begin(inode, pos, len, flags);
+
 	f2fs_balance_fs(sbi);
 repeat:
 	err = f2fs_convert_inline_data(inode, pos + len);