mtab: Use mount IDs instead of dev_t for bfs_fstype()

This fixes -fstype on btrfs subvolumes, which previously reported
"unknown" due to their dev_t being potentially different from the mount
point.

Link: https://savannah.gnu.org/bugs/?50859
Link: https://lwn.net/Articles/866582/

src/mtab.c

@@ -256,7 +256,7 @@ static int bfs_mtab_fill_types(struct bfs_mtab *mtab) {
 			continue;
 		}
 
-		if (trie_set_mem(&mtab->types, &sb.dev, sizeof(sb.dev), mount->type) != 0) {
+		if (trie_set_mem(&mtab->types, &sb.mnt_id, sizeof(sb.mnt_id), mount->type) != 0) {
 			goto fail;
 		}
 	}
@@ -279,7 +279,7 @@ const char *bfs_fstype(const struct bfs_mtab *mtab, const struct bfs_stat *statb
 		}
 	}
 
-	const char *type = trie_get_mem(&mtab->types, &statbuf->dev, sizeof(statbuf->dev));
+	const char *type = trie_get_mem(&mtab->types, &statbuf->mnt_id, sizeof(statbuf->mnt_id));
 	if (type) {
 		return type;
 	} else {

tests/gnu/fstype_btrfs_subvol.out

@@ -0,0 +1,4 @@
+mnt
+mnt/file
+mnt/subvol
+mnt/subvol/file

tests/gnu/fstype_btrfs_subvol.sh

@@ -0,0 +1,25 @@
+# Test that -fstype works in btrfs subvolumes
+
+command -v btrfs &>/dev/null || skip
+
+cd "$TEST"
+
+# Make a btrfs filesystem image
+truncate -s128M img
+mkfs.btrfs img >&2
+
+# Mount it
+mkdir mnt
+bfs_sudo mount img mnt || skip
+defer bfs_sudo umount mnt
+
+# Make it owned by us
+bfs_sudo chown "$(id -u):$(id -g)" mnt
+
+# Create a subvolume inside it
+btrfs subvolume create mnt/subvol >&2
+
+# Make a file in and outside the subvolume
+"$XTOUCH" mnt/file mnt/subvol/file
+
+bfs_diff mnt -fstype btrfs -print -o -printf '%p %F\n'