lib: net_buf: rename header file from zephyr/net/buf.h to zephyr/net_buf.h

Move the network buffer header file from zephyr/net/buf.h to
zephyr/net_buf.h as the implementation now lives outside of the networking
subsystem.

Add (deprecated) zephyr/net/buf.h header to maintain compatibility with old
file path.

Signed-off-by: Henrik Brix Andersen <henrik@brixandersen.dk>