fs/jfs: cast inactags to s64 to prevent potential overflow

The expression "inactags << bmp->db_agl2size" in the function
dbFinalizeBmap() is computed using int operands. Although the
values (inactags and db_agl2size) are derived from filesystem
parameters and are usually small, there is a theoretical risk that
the shift could overflow a 32-bit int if extreme values occur.

According to the C standard, shifting a signed 32-bit int can lead
to undefined behavior if the result exceeds its range. In our
case, an overflow could miscalculate free blocks, potentially
leading to erroneous filesystem accounting.

To ensure the arithmetic is performed in 64-bit space, we cast
"inactags" to s64 before shifting. This defensive fix prevents any
risk of overflow and complies with kernel coding best practices.

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

Signed-off-by: Rand Deeb <[e-mail]>
Signed-off-by: Dave Kleikamp <[e-mail]>

fs/jfs/jfs_dmap.c

···
 	 * system size is not a multiple of the group size).
 	 */
 	inactfree = (inactags && ag_rem) ?
-	    ((inactags - 1) << bmp->db_agl2size) + ag_rem
-	    : inactags << bmp->db_agl2size;
+	    (((s64)inactags - 1) << bmp->db_agl2size) + ag_rem
+	    : ((s64)inactags << bmp->db_agl2size);
 
 	/* determine how many free blocks are in the active
 	 * allocation groups plus the average number of free blocks