Witness: acquiring duplicate lock of same type: "ftlk" in linux_futex.c

[Pieter de Goeje]

After upgrading a kernel from 7-stable to current, I got this witness 
warning during startup:

acquiring duplicate lock of same type: "ftlk"
  1st ftlk @ 
/FreeBSD/FreeBSD-current-clean/src/sys/modules/linux/../../compat/linux/linux_futex.c:177
  2nd ftlk @ 
/FreeBSD/FreeBSD-current-clean/src/sys/modules/linux/../../compat/linux/linux_futex.c:203
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2a
_witness_debugger() at _witness_debugger+0x2e
witness_checkorder() at witness_checkorder+0x8ef
_sx_xlock() at _sx_xlock+0x55
futex_get0() at futex_get0+0xfe
linux_sys_futex() at linux_sys_futex+0x22b
ia32_syscall() at ia32_syscall+0x19c
Xint0x80_syscall() at Xint0x80_syscall+0x95
--- syscall (240, Linux ELF32, linux_sys_futex), rip = 0x28080a91, rsp = 
0x2b13834c, rbp = 0x2 ---

I run a linux app (half-life dedicated server) on linux_base-f8, which 
uses futexes. The warning doesn't seem to impact functionality.

I don't think it matters but I only installed the new kernel and modules 
to test things before I do a full upgrade.

- Pieter