fs/jfs: Fix OOB read caused by invalid dir slot index

fs/jfs: Fix OOB read caused by invalid dir slot index

While fuzz testing JFS with ASAN enabled an OOB read was detected in
grub_jfs_opendir(). The issue occurred due to an invalid directory slot
index in the first entry of the sorted directory slot array in the inode
directory header. The fix ensures the slot index is validated before
accessing it. Given that an internal or a leaf node in a directory B+
tree is a 4 KiB in size and each directory slot is always 32 bytes, the
max number of slots in a node is 128. The validation ensures that the
slot index doesn't exceed this limit.

[1] https://jfs.sourceforge.net/project/pub/jfslayout.pdf

  JFS will allocate 4K of disk space for an internal node of the B+ tree.
  An internal node looks the same as a leaf node.
          - page 10

  Fixed number of Directory Slots depending on the size of the node. These are
  the slots to be used for storing the directory slot array and the directory
  entries or router entries. A directory slot is always 32 bytes.
  ...
  A Directory Slot Array which is a sorted array of indices to the directory
  slots that are currently in use.
  ...
  An internal or a leaf node in the directory B+ tree is a 4K page.
          - page 25

Signed-off-by: Lidong Chen <lidong.chen@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Reviewed-by: Alec Brown <alec.r.brown@oracle.com>
Gbp-Pq: Topic cve-2025-jan
Gbp-Pq: Name fs-jfs-Fix-OOB-read-caused-by-invalid-dir-slot-index.patch