fs/jfs: Add check for negative db_l2nbperpage

In the Linux kernel, the following vulnerability has been resolved: fs/jfs: Add check for negative db_l2nbperpage l2nbperpage is log2(number of blks per page), and the minimum legal value should be 0, not negative. In the case of l2nbperpage being negative, an error will occur when subsequently used as shift exponent. Syzbot reported this bug: UBSAN: shift-out-of-bounds in fs/jfs/jfs_dmap.c:799:12 shift exponent -16777216 is negative