smb: client: fix zero length for mkdir POSIX create context

SMB create requests issued via smb311_posix_mkdir() have an incorrect
length of zero bytes for the POSIX create context data. ksmbd server
rejects such requests and logs "cli req too short" causing mkdir to fail
with "invalid argument" on the client side.  It also causes subsequent
rmmod to crash in cifs_destroy_request_bufs()

Inspection of packets sent by cifs.ko using wireshark show valid data for
the SMB2_POSIX_CREATE_CONTEXT is appended with the correct offset, but
with an incorrect length of zero bytes. Fails with ksmbd+cifs.ko only as
Windows server/client does not use POSIX extensions.

Fix smb311_posix_mkdir() to set req->CreateContextsLength as part of
appending the POSIX creation context to the request.

Signed-off-by: Jethro Donaldson <devel@jro.nz>
Acked-by: Paulo Alcantara (Red Hat) <pc@manguebit.com>
Reviewed-by: Namjae Jeon <linkinjeon@kernel.org>
Cc: stable@vger.kernel.org
Signed-off-by: Steve French <stfrench@microsoft.com>