ext4: fix the number of credits needed for ext4_ext_migrate()

The migration ioctl creates a temporary inode.  Since this inode is
never linked to a directory, we don't need to reserve journal credits
required for modifying the directory.

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