maximum limit of files open with O_EXLOCK

[nia <nia%NetBSD.org@localhost>]

per https://github.com/ziglang/zig/issues/9162,

There seems to be a limit somewhere that causes open() to return ENOMEM
when 1000+ files are opened with O_EXLOCK, this is independent of any
"open file" limits.

I eyeballed the vfs syscalls code but couldn't figure out where
ENOMEM is coming from. The problem seems filesystem independent - I
tried with tmpfs and ZFS and got the same result with both.

Here's a simple reproducer:

#include <sys/param.h>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>

int main(int argc, char **argv ){ 
	char path[32];

	for (int i = 0; i < 2000; ++i) {
		snprintf(path, sizeof(path), "/tmp/XXX%d", i);	
		int n = open(path, O_CREAT | O_EXLOCK);
		if (n == -1) {
			printf("bleh %d: %s\n", i, strerror(errno));
			return 1;
		}
	}
	return 0;
}

$ ./a.out
bleh 1010: Cannot allocate memory

While it's arguable that what Zig is doing here isn't sensible,
it would be nice to know where the limit is coming from and whether
it's adjustable.