Revert "net: change accept_ra_min_rtr_lft to affect all RA lifetimes"

This reverts commit bad004c3 which is
commit 5027d54a upstream.

It breaks the Android ABI, and is already merged in the non-LTS branch
in an abi-safe way.

Bug: 161946584
Change-Id: I9f0f611a3ecbae0ae154ae465ff7472d8e98a1d8
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>