srcu: Make Tiny SRCU able to operate in preemptible kernels

srcu: Make Tiny SRCU able to operate in preemptible kernels

Given that SRCU allows its read-side critical sections are not just
preemptible, but also allow general blocking, there is not much
reason to restrict Tiny SRCU to non-preemptible kernels.  This commit
therefore removes Tiny SRCU dependencies on non-preemptibility, primarily
surrounding its interaction with rcutorture and early boot.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Ankur Arora <ankur.a.arora@oracle.com>
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>