netfilter: nfnetlink: nfnetlink_unicast() reports EAGAIN instead of ENOBUFS

Frontend callback reports EAGAIN to nfnetlink to retry a command, this
is used to signal that module autoloading is required. Unfortunately,
nlmsg_unicast() reports EAGAIN in case the receiver socket buffer gets
full, so it enters a busy-loop.

This patch updates nfnetlink_unicast() to turn EAGAIN into ENOBUFS and
to use nlmsg_unicast(). Remove the flags field in nfnetlink_unicast()
since this is always MSG_DONTWAIT in the existing code which is exactly
what nlmsg_unicast() passes to netlink_unicast() as parameter.

Fixes: 96518518 ("netfilter: add nftables")
Reported-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>