Preallocate some slices

Fix *some* of the prealloc linter warnings. While it does not make sense
to address all warnings (or add prealloc to the list of linters we run
in CI), some do make sense.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

events.go

@@ -199,7 +199,7 @@ func convertMemoryEntry(c cgroups.MemoryData) types.MemoryEntry {
 }
 
 func convertBlkioEntry(c []cgroups.BlkioStatEntry) []types.BlkioEntry {
-	var out []types.BlkioEntry
+	out := make([]types.BlkioEntry, 0, len(c))
 	for _, e := range c {
 		out = append(out, types.BlkioEntry(e))
 	}

libcontainer/internal/userns/usernsfd_linux.go

@@ -41,10 +41,11 @@ func (m Mapping) toSys() (uids, gids []syscall.SysProcIDMap) {
 // the uid and gid mappings (because the order doesn't matter to the kernel).
 // The set of userns handles is indexed using this ID.
 func (m Mapping) id() string {
-	var uids, gids []string
+	uids := make([]string, 0, len(m.UIDMappings))
 	for _, idmap := range m.UIDMappings {
 		uids = append(uids, fmt.Sprintf("%d:%d:%d", idmap.ContainerID, idmap.HostID, idmap.Size))
 	}
+	gids := make([]string, 0, len(m.GIDMappings))
 	for _, idmap := range m.GIDMappings {
 		gids = append(gids, fmt.Sprintf("%d:%d:%d", idmap.ContainerID, idmap.HostID, idmap.Size))
 	}