fs/jfs: Prevent integer overflow in AG size calculation

In the Linux kernel, the following vulnerability has been resolved:

fs/jfs: Prevent integer overflow in AG size calculation

The JFS filesystem calculates allocation group (AG) size using 1 <<

l2agsize in dbExtendFS(). When l2agsize exceeds 31 (possible with >2TB

aggregates on 32-bit systems), this 32-bit shift operation causes undefined

behavior and improper AG sizing.

On 32-bit architectures:

- Left-shifting 1 by 32+ bits results in 0 due to integer overflow

- This creates invalid AG sizes (0 or garbage values) in

sbi->bmap->db_agsize

- Subsequent block allocations would reference invalid AG structures

- Could lead to:

- Filesystem corruption during extend operations

- Kernel crashes due to invalid memory accesses

- Security vulnerabilities via malformed on-disk structures

Fix by casting to s64 before shifting:

bmp->db_agsize = (s64)1 << l2agsize;

This ensures 64-bit arithmetic even on 32-bit architectures. The cast

matches the data type of db_agsize (s64) and follows similar patterns in

JFS block calculation code.

Found by Linux Verification Center (linuxtesting.org) with SVACE.