zed: only go up to current limit in close_from() fallback

Consider the following strace log:
  prlimit64(0, RLIMIT_NOFILE,
            NULL, {rlim_cur=1024, rlim_max=1024*1024}) = 0
  dup2(0, 30)                         = 30
  dup2(0, 300)                        = 300
  dup2(0, 3000)                       = -1 EBADF (Bad file descriptor)
  dup2(0, 30000)                      = -1 EBADF (Bad file descriptor)
  dup2(0, 300000)                     = -1 EBADF (Bad file descriptor)
  prlimit64(0, RLIMIT_NOFILE,
            {rlim_cur=1024*1024, rlim_max=1024*1024}, NULL) = 0
  dup2(0, 30)                         = 30
  dup2(0, 300)                        = 300
  dup2(0, 3000)                       = 3000
  dup2(0, 30000)                      = 30000
  dup2(0, 300000)                     = 300000

Even a privileged process needs to bump its rlimit before being able
to use fds higher than rlim_cur.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #11834

cmd/zed/zed_file.c

@@ -17,7 +17,6 @@
 #include <fcntl.h>
 #include <limits.h>
 #include <string.h>
-#include <sys/resource.h>
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <unistd.h>
@@ -175,9 +174,7 @@ zed_file_is_locked(int fd)
 void
 zed_file_close_from(int lowfd)
 {
-	static const int maxfd_def = 256;
 	int errno_bak = errno;
-	struct rlimit rl;
 	int maxfd = 0;
 	int fd;
 	DIR *fddir;
@@ -190,11 +187,8 @@ zed_file_close_from(int lowfd)
 				maxfd = fd;
 		}
 		(void) closedir(fddir);
-	} else if (getrlimit(RLIMIT_NOFILE, &rl) < 0 ||
-	    rl.rlim_max == RLIM_INFINITY) {
-		maxfd = maxfd_def;
 	} else {
-		maxfd = rl.rlim_max;
+		maxfd = sysconf(_SC_OPEN_MAX);
 	}
 	for (fd = lowfd; fd < maxfd; fd++)
 		(void) close(fd);