ext4: fix setting of referenced bit in ext4_es_lookup_extent()

We were setting referenced bit on the extent structure we return from
ext4_es_lookup_extent() which is just a private structure on stack. Thus
setting had no effect. Set the bit in the structure in the status tree
instead.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>