ext4: Don't set EXTENTS_FL flag for fast symlinks

For fast symbolic links, the file content is stored in the i_block[]
array, which is not compatible with the new file extents format.
e2fsck reports error on such files because EXTENTS_FL is set.
Don't set the EXTENTS_FL flag when creating fast symlinks.

In the case of file migration, skip fast symbolic links.

Signed-off-by: Valerie Clement <valerie.clement@bull.net>
Signed-off-by: Mingming Cao <cmm@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>