fs/jfs: Fix OOB read in jfs_getent()

fs/jfs: Fix OOB read in jfs_getent()

The JFS fuzzing revealed an OOB read in grub_jfs_getent(). The crash
was caused by an invalid leaf nodes count, diro->dirpage->header.count,
which was larger than the maximum number of leaf nodes allowed in an
inode. This fix is to ensure that the leaf nodes count is validated in
grub_jfs_opendir() before calling grub_jfs_getent().

On the occasion replace existing raw numbers with newly defined constant.

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>