nvmet-tcp: fix callback lock for TLS handshake

When restoring the default socket callbacks during a TLS handshake, we
need to acquire a write lock on sk_callback_lock.  Previously, a read
lock was used, which is insufficient for modifying sk_user_data and
sk_data_ready.

Fixes: 675b453e ("nvmet-tcp: enable TLS handshake upcall")
Signed-off-by: Maurizio Lombardi <mlombard@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>