sunrpc: exclude from freezer when waiting for requests:

Prior to v6.1, the freezer will only wake a kernel thread from an
uninterruptible sleep.  Since we changed svc_get_next_xprt() to use and
IDLE sleep the freezer cannot wake it.  We need to tell the freezer to
ignore it instead.

To make this work with only upstream commits, 5.15.y would need
commit f5d39b02 ("freezer,sched: Rewrite core freezer logic")
which allows non-interruptible sleeps to be woken by the freezer.

Fixes: 9b8a8e5e ("nfsd: don't allow nfsd threads to be signalled.")
Tested-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>