linuxkpi: Add <linux/dma-buf-map.h>

I took the implementation from OpenBSD, commit
d55ef580b1748517027c3eabdb715316ca5b1442.

The only difference is the addition of dma_buf_map_is_equal().

Reviewed by: manu
Approved by: manu
Differential Revision: https://reviews.freebsd.org/D36963