f2fs: disable extent_cache for fcollapse/finsert inodes

This reduces the elapsed time to do xfstests/generic/017.

Before: 458 s
After:  390 s

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
index b1dc972..72e52cd 100644
--- a/fs/f2fs/file.c
+++ b/fs/f2fs/file.c
@@ -1044,6 +1044,9 @@
 
 	f2fs_balance_fs(sbi, true);
 	f2fs_lock_op(sbi);
+
+	f2fs_drop_extent_tree(inode);
+
 	ret = __exchange_data_block(inode, inode, end, start, nrpages - end, true);
 	f2fs_unlock_op(sbi);
 	return ret;
@@ -1278,6 +1281,8 @@
 		idx -= nr;
 
 		f2fs_lock_op(sbi);
+		f2fs_drop_extent_tree(inode);
+
 		ret = __exchange_data_block(inode, inode, idx,
 					idx + delta, nr, false);
 		f2fs_unlock_op(sbi);