file: /proc/sys/net/ipv4/conf/interface/xfrm4_gc_thresh
variable: net.ipv4.conf.interface.xfrm4_gc_thresh
Official reference

The threshold at which we will start garbage collecting for IPv4 destination cache entries. At twice this value the system will refuse new allocations.

Nb: per interface setting (where “interface” is the name of your network interface); “all” is a special interface: changes the settings for all interfaces.