rcutorture: Make cur_ops->format_gp_seqs take buffer length

The Tree and Tiny implementations of rcutorture_format_gp_seqs() use
hard-coded constants for the length of the buffer that they format into.
This is of course an accident waiting to happen, so this commit therefore
makes them take a length argument.  The rcutorture calling code uses
ARRAY_SIZE() to safely compute this new argument.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>