nfsd: safer handling of corrupted c_type

[ Upstream commit c25bf185 ]

This can only happen if there's a bug somewhere, so let's make it a WARN
not a printk.  Also, I think it's safest to ignore the corruption rather
than trying to fix it by removing a cache entry.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>