Install prerequisites:

yum groupinstall "development tools" 
yum install kernel-devel kernel-headers ncurses-devel
yum install xmlto asciidoc python-devel newt-devel perl pesign elfutils-devel audit-libs-devel perl-ExtUtils-Embed.noarch

Download and install the kernel source:

cd /root
yumdownloader --source kernel
yum install kernel-3.10.0-229.4.2.el7.src.rpm

Prepare for building a binary RPM. Make sure you have enough space on the file system:

cd /root/rpmbuild/SPECS/
rpmbuild -bp kernel.spec

Configure the kernel module:

cd /root/rpmbuild/BUILD/kernel-3.10.0-229.4.2.el7/linux-3.10.0-229.4.2.el7.centos.x86_64
make menuconfig

Go to 'Device Drivers | Block devices' and set 'M' on 'Network block device support' exit saving new kernel config.

Compile time (get some coffee):

make prepare && make modules_prepare -j4 && make -j4

Next build the block device modules:

make M=drivers/block

Check the module:

modinfo drivers/block/nbd.ko

If all looks good copy it to the running kernels extra folder in /lib/modules/$(uname -r)/extra.

Load the module:

depmod -a && modprobe nbd