schfs: several snapshots at once, and unmount gives the slot back

Two loose ends from the mount-by-name commit, both closed.

A slot is a device number, so supporting more than one snapshot is a
small array rather than a redesign: SCHFS_SNAP_DEV through
SCHFS_SNAP_DEV + SCHFS_SNAP_SLOTS - 1, each with its own volume and its
own scratch pages, allocated on first use. Four of them -- "one at a
time" is an odd thing to have to explain, and the cost is two pages per
slot. Not many, because each one is a device number and those are a
small fixed space.

The leak mattered more. umount removed the mount table entry and left the
slot marked in use, so a snapshot could be unmounted and then never
mounted again until reboot. sys_umount2 releases it now, keyed off the
device it already has in hand.

Verified in the running system: two snapshots mounted at once, both
readable, both unmounted, and a third mount afterwards succeeds and reads
correctly -- which is the case that failed before.

schfs-check passes; bootcheck 108 [ok].

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

kernel/fs/schfs/vfs.c

@@ -109,7 +109,7 @@ schfs_vfs_iload(struct inode *ip)
 static int
 readonly(const struct inode *ip)
 {
-  return ip->dev == SCHFS_SNAP_DEV;
+  return SCHFS_IS_SNAP_DEV(ip->dev);
 }
 
 static void