smb/server: fix potential null-ptr-deref of lease_ctx_info in smb2_open()

In the Linux kernel, the following vulnerability has been resolved: smb/server: fix potential null-ptr-deref of lease_ctx_info in smb2_open() null-ptr-deref will occur when (req_op_level == SMB2_OPLOCK_LEVEL_LEASE) and parse_lease_state() return NULL. Fix this by check if 'lease_ctx_info' is NULL. Additionally, remove the redundant parentheses in parse_durable_handle_context().