ext4: fix leak in check_filename()

root_first_block_buffer should be free()d in all cases, not just when an
error occurs. Fix the success exit path of the function to do this.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Lukasz Majewski <l.majewski@samsung.com>
Tested-by: Lukasz Majewski <l.majewski@samsung.com>