ext4: set extents flag when migrating file to use extents

Fix a typo that was introduced in commit 07a03824

 (in 2.6.36) which
caused the extents flag not to be set at the conclusion of converting
an inode to use extents.

Reported-by: Peter Uchno <peter.uchno@gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>