Revert "ipv4: Make INET_LRO a bool instead of tristate."

This reverts commit e81963b1.

LRO is now deprecated in favour of GRO, and only a few drivers use it,
so it is desirable to build it as a module in distribution kernels.

The original change to prevent building it as a module was made in an
attempt to avoid the case where some dependents are set to y and some
to m, and INET_LRO can be set to m rather than y.  However, the
Kconfig system will reliably set INET_LRO=y in this case.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>