srcu: Move SRCU Tree/Tiny definitions from srcu.h

There are a couple of definitions under "#ifdef CONFIG_TINY_SRCU"
in include/linux/srcu.h.  There is no point in them being there,
so this commit moves them to include/linux/srcutiny.h and
include/linux/srcutree.c, thus eliminating that #ifdef.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: <bpf@vger.kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>