smb: smbdirect: let smbdirect_connection_deregister_mr_io unlock while waiting

smb: smbdirect: let smbdirect_connection_deregister_mr_io unlock while waiting

We should not hold a mutex locked during wait_for_completion()
holding a reference is enough.

Cc: Steve French <smfrench@gmail.com>
Cc: Tom Talpey <tom@talpey.com>
Cc: Long Li <longli@microsoft.com>
Cc: Namjae Jeon <linkinjeon@kernel.org>
Cc: David Howells <dhowells@redhat.com>
Cc: Henrique Carvalho <henrique.carvalho@suse.com>
Cc: linux-cifs@vger.kernel.org
Cc: samba-technical@lists.samba.org
Signed-off-by: Stefan Metzmacher <metze@samba.org>
Signed-off-by: Steve French <stfrench@microsoft.com>