fs : Fix warning using plain integer as NULL

Sparse static analysis tools generate a warning with this message
"Using plain integer as NULL pointer". In this case this warning is
being shown because we are trying to initialize  pointer to NULL using
integer value 0.

Signed-off-by: Abhinav Singh <singhabhinav9051571833@gmail.com>
Link: https://lore.kernel.org/r/20231108044550.1006555-1-singhabhinav9051571833@gmail.com


Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>