Raise the number of nested readers limit 2^16 and 2^32 on 32 and 64-bits arch.

Raise the number of nested readers limit 2^16 and 2^32 on 32 and 64-bits arch.

> I don't know of any code in the Linux kernel that nests rcu_read_lock()
> anywhere near that deep.  And if someone does find such a case, it is
> pretty easy to use 15 bits rather than 8 to hold the nesting depth, just
> by changing the definition of RCU_GP_CTR_BIT.
>

You know what ? Changing RCU_GP_CTR_BIT to 16 uses a
testw %ax, %ax instead of a testb %al, %al. The trick here is that
RCU_GP_CTR_BIT must be a multiple of 8 so we can use a full 8-bits,
16-bits or 32-bits bitmask for the lower order bits.

On 64-bits, using a RCU_GP_CTR_BIT of 32 is also ok. It uses a testl.

To provide 32-bits compability and allow the deepest nesting possible, I
think it makes sense to use

/* Use the amount of bits equal to half of the architecture long size */

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>