fs/jfs: Add validity check for db_maxag and db_agpref

In the Linux kernel, the following vulnerability has been resolved: fs/jfs: Add validity check for db_maxag and db_agpref Both db_maxag and db_agpref are used as the index of the db_agfree array, but there is currently no validity check for db_maxag and db_agpref, which can lead to errors. The following is related bug reported by Syzbot: UBSAN: array-index-out-of-bounds in fs/jfs/jfs_dmap.c:639:20 index 7936 is out of range for type 'atomic_t[128]' Add checking that the values of db_maxag and db_agpref are valid indexes for the db_agfree array.