ext4/306: Fixup resize2fs usage

Use $RESIZE2FS_PROG instead of direct name to call program with full
path. Also add an appropriate feature test.

Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>