homed: add env var for overriding default mount options

This adds an esay way to override the default mount options to use for
LUKS home dirs via the env vars SYSTEMD_HOME_MOUNT_OPTIONS_EXT4,
SYSTEMD_HOME_MOUNT_OPTIONS_BTRFS, SYSTEMD_HOME_MOUNT_OPTIONS_XFS.

See: #15120

src/home/homework-mount.c

@@ -20,6 +20,17 @@
 #include "user-util.h"
 
 static const char *mount_options_for_fstype(const char *fstype) {
+        const char *e;
+        char *n;
+
+        assert(fstype);
+
+        /* Allow overriding our built-in defaults with an environment variable */
+        n = strjoina("SYSTEMD_HOME_MOUNT_OPTIONS_", fstype);
+        e = getenv(ascii_strupper(n));
+        if (e)
+                return e;
+
         if (streq(fstype, "ext4"))
                 return "noquota,user_xattr";
         if (streq(fstype, "xfs"))