netlink: Introduce macro NETLINK_MSG_APPEND to wrap nlmsg_append
netlink: Introduce macro NETLINK_MSG_APPEND to wrap nlmsg_append

Introduce a macro NETLINK_MSG_APPEND to wrap nlmsg_append and
simplify code. Remove those labels 'buffer_too_small', since they
are now useless.

Signed-off-by: Shi Lei <79e2d1a953a210f9f0c36cafbe946414d6421d20@massclouds.com>
Reviewed-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>

libvirt/libvirt,crobinso/libvirt,zippy2/libvirt,libvirt/libvirt,zippy2/libvirt,nertpinx/libvirt,libvirt/libvirt,olafhering/libvirt,olafhering/libvirt,nertpinx/libvirt,nertpinx/libvirt,jfehlig/libvirt,olafhering/libvirt,libvirt/libvirt,nertpinx/libvirt,jfehlig/libvirt,zippy2/libvirt,jfehlig/libvirt,zippy2/libvirt,crobinso/libvirt,nertpinx/libvirt,crobinso/libvirt,crobinso/libvirt,olafhering/libvirt,jfehlig/libvirt