linuxkpi: Accept NULL as a value in `linux_xarray`

linuxkpi: Accept NULL as a value in `linux_xarray`

Linux' XArray allows to store a NULL pointer as a value. `xa_load()`
would return NULL for both an unused index and an index set to NULL. But
it impacts `xa_alloc()` which needs to find the next available index.

However, our implementation relies on a radix tree (see `linux_radix.c`)
which does not accept NULL pointers as values. I'm not sure if this is a
limitation or a feature, so to work around this, a NULL value is
replaced by `NULL_VALUE`, an unlikely address, when we pass it to
linux_radix.

Reviewed by:	emaste, manu
Approved by:	emaste, manu
Differential Revision:	https://reviews.freebsd.org/D38543

(cherry picked from commit 3102ea3b15b6c3ed1ea50716d65980b680375ebc)