CIFS: fix max ea value size

It should not be larger then the slab max buf size. If user
specifies a larger size, it passes this check and goes
straightly to SMB2_set_info_init performing an insecure memcpy.

Signed-off-by: Murphy Zhou <jencce.kernel@gmail.com>
Reviewed-by: Aurelien Aptel <aaptel@suse.com>
CC: Stable <stable@vger.kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>