dma-buf: Fix ifnullfree.cocci warnings

NULL check before some freeing functions is not needed.
drivers/dma-buf/dma-buf.c:1183:2-26: WARNING: NULL check before freeing debugfs_remove_recursive
Generated by: scripts/coccinelle/free/ifnullfree.cocci

Signed-off-by: Vasyl Gomonovych <gomonovych@gmail.com>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1511364161-6074-1-git-send-email-gomonovych@gmail.com