nfsd: fix net-namespace logic in __nfsd_file_cache_purge

If the namespace doesn't match the one in "net", then we'll continue,
but that doesn't cause another rhashtable_walk_next call, so it will
loop infinitely.

Fixes: ce502f81 ("NFSD: Convert the filecache to use rhashtable")
Reported-by: Petr Vorel <pvorel@suse.cz>
Link: https://lore.kernel.org/ltp/Y1%2FP8gDAcWC%2F+VR3@pevik/


Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>