fs: jfs: fix shift-out-of-bounds in dbAllocAG

In the Linux kernel, the following vulnerability has been resolved: fs: jfs: fix shift-out-of-bounds in dbAllocAG Syzbot found a crash : UBSAN: shift-out-of-bounds in dbAllocAG. The underlying bug is the missing check of bmp->db_agl2size. The field can be greater than 64 and trigger the shift-out-of-bounds. Fix this bug by adding a check of bmp->db_agl2size in dbMount since this field is used in many following functions. The upper bound for this field is L2MAXL2SIZE - L2MAXAG, thanks for the help of Dave Kleikamp. Note that, for maintenance, I reorganized error handling code of dbMount.