fs/jfs: Prevent integer overflow in AG size calculation

[ Upstream commit 7fcbf78 ]

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.

Signed-off-by: Rand Deeb <rand.sec96@gmail.com>
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

fs/jfs/jfs_dmap.c

@@ -3403,7 +3403,7 @@ int dbExtendFS(struct inode *ipbmap, s64 blkno,	s64 nblocks)
 	oldl2agsize = bmp->db_agl2size;
 
 	bmp->db_agl2size = l2agsize;
-	bmp->db_agsize = 1 << l2agsize;
+	bmp->db_agsize = (s64)1 << l2agsize;
 
 	/* compute new number of AG */
 	agno = bmp->db_numag;