util: Add Free BSD Red-Black tree macros

Copy Free BSD's sys/sys/tree.h to SPDK's include/spdk/tree.h by
changing as little as possible.

A few changes are as follows:
- Remove a few lines located at the head of the file.
- Change the name of ifdef from _SYS_TREE_H_ to SPDK_TREE_H.
- Change the type __uintptr_t to uintptr_t.
- Change the attribute __unused to __attribute__((unused))
- Insert a space to the front and the end of CONSTCOND.

Signed-off-by: Shuhei Matsumoto <[email protected]>
Change-Id: I26488d8fd61f80508cf7d96fbeb6007f12080a01
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/7721
Tested-by: SPDK CI Jenkins <[email protected]>
Community-CI: Mellanox Build Bot
Reviewed-by: Aleksey Marchuk <[email protected]>
Reviewed-by: Ben Walker <[email protected]>
Reviewed-by: Jim Harris <[email protected]>
Reviewed-by: Monica Kenguva <[email protected]>
Reviewed-by: Konrad Sztyber <[email protected]>