filesystems: add internal APIs to convert fs magic to name

We previously had tooling for the opposite direction, let's complete the
work.

src/basic/generate-filesystem-switch-case.py

@@ -0,0 +1,63 @@
+#!/usr/bin/env python3
+# SPDX-License-Identifier: LGPL-2.1-or-later
+
+import sys
+
+
+def filter_fsname(name):
+    # File system magics are sometimes not unique, because file systems got new
+    # revisions or got renamed. Let's prefer newer over older here, and thus
+    # ignore the old names. Specifically:
+    #
+    # → cgroupfs took over the magic of cpuset
+    # → devtmpfs is not a file system of its own, but just a "named superblock" of tmpfs
+    # → ext4 is the newest revision of ext2 + ext3
+    # → fuseblk is closely related to fuse, so close that they share a single magic, but the latter is more common
+    # → gfs2 is the newest revision of gfs
+    # → vfat is the newest revision of msdos
+    # → ncpfs (not ncp) was the last name of the netware `file_system_type` name before it was removed in 2018
+    # → nfs4 is the newest revision of nfs
+    # → orangefs is the new name of pvfs2
+    # → smb3 is an alias for cifs
+
+    return name in (
+        "cpuset",
+        "devtmpfs",
+        "ext2",
+        "ext3",
+        "fuseblk",
+        "gfs",
+        "msdos",
+        "ncp",
+        "nfs",
+        "pvfs2",
+        "smb3",
+    )
+
+
+gperf_file = sys.argv[1]
+keywords_section = False
+
+for line in open(gperf_file):
+    if line[0] == "#":
+        continue
+
+    if keywords_section:
+        name, ids = line.split(",", 1)
+
+        name = name.strip()
+        if filter_fsname(name):
+            continue
+
+        ids = ids.strip()
+        assert ids[0] == "{"
+        assert ids[-1] == "}"
+        ids = ids[1:-1]
+
+        for id in ids.split(","):
+            print(f"case (statfs_f_type_t) {id.strip()}:")
+
+        print(f'        return "{name}";')
+
+    if line.startswith("%%"):
+        keywords_section = True