f2fs: allow readdir() to be interrupted

This patch follows ext4 to allow readdir() in large empty directory to
be interrupted. Referenced commit of ext4: 1f60fbe7 ("ext4: allow
readdir()'s of large empty directories to be interrupted").

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>