Bump Linux kernel to 5.10

This bumps our Linux kernel to 5.10. There's one minor fix in fsinfo accounting for the fact that strings are
now null-terminated. While debugging this I also drive-by fixed a minor typing issue in quotactl.go.

This drops support for the old initramfs loading method (which was the driving force for the EROFS changes)
as refactors in the kernel made the patch we carried until now non-viable. Nothing uses it anymore, everything is
either a microvm-style machine which doesn't use EFI and thus doesn't suffer from the issue or uses EROFS.

Test Plan: No new functionality, should be covered by E2E tests.

X-Origin-Diff: phab/D697
GitOrigin-RevId: d8e40954abb66cb082eecbca372b94a7e40b84a8
diff --git a/WORKSPACE b/WORKSPACE
index 62762e8..98692b3 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -105,7 +105,7 @@
 
 linux_external(
     name = "linux",
-    version = "5.6",
+    version = "5.10.4",
 )
 
 load("//third_party/edk2:external.bzl", "edk2_external")