CSan issues in sys_execve()

I already reported one CSan issue on NetBSD 10, here is the second one.

At first, I compiled the kernel with a different sanitizer (KASAN)
and I noticed that it reports a problem in elf64_copyargs if I run

./build.sh -j 40 release (plus other flags)

I since switched to KCSAN and it also reports potential issues but more
frequently, it triggers even if I run ./build.sh -j 16 cleandir:

CSan: Racy Access [Cpu0 Write Addr=0xffffbaa110270000 Size=262144 PC=0xffffffff80d2ce10<execve_loadvm>] [Cpu2 Read Addr=0xffffbaa110230071 Size=262144 PC=0xffffffff80d2e09c<copyargs>]
#0 0xffffffff80d6ea6d in kcsan_copyinstr <netbsd>
#1 0xffffffff80d2ce10 in execve_loadvm <netbsd>
#2 0xffffffff80d2c701 in sys_execve <netbsd>
#3 0xffffffff8059a193 in syscall <netbsd>
CSan: Racy Access [Cpu0 Read Addr=0xffffbaa110230011 Size=262144 PC=0xffffffff80d2e09c<copyargs>] [Cpu1 Write Addr=0xffffbaa11027000f Size=262129 PC=0xffffffff80d2d0f0<execve_loadvm>]
#0 0xffffffff80d6eab7 in kcsan_copyoutstr <netbsd>
#1 0xffffffff80d2e09c in copyargs <netbsd>
#2 0xffffffff80d18708 in elf64_copyargs <netbsd>
#3 0xffffffff80d2d8cf in execve_runproc <netbsd>
#4 0xffffffff80d2c71c in sys_execve <netbsd>
#5 0xffffffff8059a193 in syscall <netbsd>

--
Alex