dma-debug: make __dma_entry_alloc_check_leak() static

Fix the following sparse warning:

kernel/dma/debug.c:659:6: warning: symbol '__dma_entry_alloc_check_leak'
was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>