btrfs-util,tmpfiles: cleanup use of ERRNO_IS_NOT_SUPPORTED()

Given that ERRNO_IS_NOT_SUPPORTED() also matches positive values,
make sure this macro is not called with arguments that do not have
errno semantics.

In this case the arguments passed to ERRNO_IS_NOT_SUPPORTED() are the
values returned by btrfs_subvol_make_fd() which is not expected to
return any positive values, but let's be consistent anyway and move
ERRNO_IS_NOT_SUPPORTED() invocations to the branches where
the return values are known to be negative.

src/shared/btrfs-util.c

@@ -1565,14 +1565,16 @@ int btrfs_subvol_snapshot_at_full(
                         return -EISDIR;
 
                 r = btrfs_subvol_make_fd(new_fd, subvolume);
-                if (ERRNO_IS_NOT_SUPPORTED(r) && (flags & BTRFS_SNAPSHOT_FALLBACK_DIRECTORY)) {
-                        /* If the destination doesn't support subvolumes, then use a plain directory, if that's requested. */
-                        if (mkdirat(new_fd, subvolume, 0755) < 0)
-                                return -errno;
+                if (r < 0) {
+                        if (ERRNO_IS_NOT_SUPPORTED(r) && (flags & BTRFS_SNAPSHOT_FALLBACK_DIRECTORY)) {
+                                /* If the destination doesn't support subvolumes, then use a plain directory, if that's requested. */
+                                if (mkdirat(new_fd, subvolume, 0755) < 0)
+                                        return -errno;
 
-                        plain_directory = true;
-                } else if (r < 0)
-                        return r;
+                                plain_directory = true;
+                        } else
+                                return r;
+                }
 
                 if (FLAGS_SET(flags, BTRFS_SNAPSHOT_LOCK_BSD)) {
                         subvolume_fd = xopenat_lock(new_fd, subvolume,

src/tmpfiles/tmpfiles.c

@@ -1883,7 +1883,7 @@ static int create_directory_or_subvolume(
         } else
                 r = 0;
 
-        if (!subvol || ERRNO_IS_NOT_SUPPORTED(r))
+        if (!subvol || (r < 0 && ERRNO_IS_NOT_SUPPORTED(r)))
                 WITH_UMASK(0000)
                         r = mkdirat_label(pfd, bn, mode);