unix/linux: update Dockerfile to Kernel 5.11, glibc 2.33, Go 1.16

The FS_POLICY_FLAGS_VALID const is kernel-internal and was removed from
the UAPI headers in
https://kernel.googlesource.com/pub/scm/linux/kernel/git/torvalds/linux/+/3ceb6543e9cf6ed87cc1fbc6f23ca2db903564cd

Change-Id: I21b1efe895fbd7f640a7604805cf7ce39c5a43f4
Reviewed-on: https://go-review.googlesource.com/c/sys/+/291637
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Trust: Matt Layher <mdlayher@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matt Layher <mdlayher@gmail.com>

unix/zerrors_linux_arm.go

@@ -163,6 +163,7 @@ const (
 	PERF_EVENT_IOC_SET_OUTPUT        = 0x2405
 	PPPIOCATTACH                     = 0x4004743d
 	PPPIOCATTCHAN                    = 0x40047438
+	PPPIOCBRIDGECHAN                 = 0x40047435
 	PPPIOCCONNECT                    = 0x4004743a
 	PPPIOCDETACH                     = 0x4004743c
 	PPPIOCDISCONN                    = 0x7439
@@ -190,6 +191,7 @@ const (
 	PPPIOCSPASS                      = 0x40087447
 	PPPIOCSRASYNCMAP                 = 0x40047454
 	PPPIOCSXASYNCMAP                 = 0x4020744f
+	PPPIOCUNBRIDGECHAN               = 0x7434
 	PPPIOCXFERUNIT                   = 0x744e
 	PR_SET_PTRACER_ANY               = 0xffffffff
 	PTRACE_GETCRUNCHREGS             = 0x19
@@ -275,6 +277,7 @@ const (
 	SO_BROADCAST                     = 0x6
 	SO_BSDCOMPAT                     = 0xe
 	SO_BUSY_POLL                     = 0x2e
+	SO_BUSY_POLL_BUDGET              = 0x46
 	SO_CNX_ADVICE                    = 0x35
 	SO_COOKIE                        = 0x39
 	SO_DETACH_REUSEPORT_BPF          = 0x44
@@ -297,6 +300,7 @@ const (
 	SO_PEERCRED                      = 0x11
 	SO_PEERGROUPS                    = 0x3b
 	SO_PEERSEC                       = 0x1f
+	SO_PREFER_BUSY_POLL              = 0x45
 	SO_PROTOCOL                      = 0x26
 	SO_RCVBUF                        = 0x8
 	SO_RCVBUFFORCE                   = 0x21