f2fs: fix to tag FIEMAP_EXTENT_MERGED in f2fs_fiemap()

f2fs does not natively support extents in metadata, 'extent' in f2fs
is used as a virtual concept, so in f2fs_fiemap() interface, it needs
to tag FIEMAP_EXTENT_MERGED flag to indicated the extent status is a
result of merging.

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