ext4: fix a typo in a comment

s/extnets/extents/

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/20200503200647.154701-1-christophe.jaillet@wanadoo.fr


Signed-off-by: Theodore Ts'o <tytso@mit.edu>