svcrpc: fix double-free on shutdown of nfsd after changing pool mode

The pool_to and to_pool fields of the global svc_pool_map are freed on
shutdown, but are initialized in nfsd startup only in the
SVC_POOL_PERCPU and SVC_POOL_PERNODE cases.

They *are* initialized to zero on kernel startup.  So as long as you use
only SVC_POOL_GLOBAL (the default), this will never be a problem.

You're also OK if you only ever use SVC_POOL_PERCPU or SVC_POOL_PERNODE.

However, the following sequence events leads to a double-free:

	1. set SVC_POOL_PERCPU or SVC_POOL_PERNODE
	2. start nfsd: both fields are initialized.
	3. shutdown nfsd: both fields are freed.
	4. set SVC_POOL_GLOBAL
	5. start nfsd: the fields are left untouched.
	6. shutdown nfsd: now we try to free them again.

Step 4 is actually unnecessary, since (for some bizarre reason), nfsd
automatically resets the pool mode to SVC_POOL_GLOBAL on shutdown.

Cc: stable@kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>