zed-functions.sh: zed_lock(): don't truncate lock

By appending instead of truncating, we can lock on any file (with write
permissions) instead of only dedicated lock files, since the locking
process itself no longer alters the file in any way

Reviewed-by: Tony Hutter <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Ahelenia Ziemiańska <[email protected]>
Closes #12042

cmd/zed/zed.d/zed-functions.sh

@@ -126,7 +126,7 @@ zed_lock()
 
     # Obtain a lock on the file bound to the given file descriptor.
     #
-    eval "exec ${fd}> '${lockfile}'"
+    eval "exec ${fd}>> '${lockfile}'"
     if ! err="$(flock --exclusive "${fd}" 2>&1)"; then
         zed_log_err "failed to lock \"${lockfile}\": ${err}"
     fi