debugfs show actual source in /proc/mounts

debugfs show actual source in /proc/mounts

After its conversion to the new mount API, debugfs displays "none" in
/proc/mounts instead of the actual source.  Fix this by recognising its
"source" mount option.

Signed-off-by: Marc Aurèle La France <tsi@tuyoix.net>
Link: https://lore.kernel.org/r/e439fae2-01da-234b-75b9-2a7951671e27@tuyoix.net
Fixes: a20971c18752 ("vfs: Convert debugfs to use the new mount API")
Cc: stable@vger.kernel.org # 6.10.x: 49abee5991e1: debugfs: Convert to new uid/gid option parsing helpers
Signed-off-by: Christian Brauner <brauner@kernel.org>