jfs: fix out-of-bounds in dbNextAG() and diAlloc()

In dbNextAG() , there is no check for the case where bmp->db_numag is
greater or same than MAXAG due to a polluted image, which causes an
out-of-bounds. Therefore, a bounds check should be added in dbMount().

And in dbNextAG(), a check for the case where agpref is greater than
bmp->db_numag should be added, so an out-of-bounds exception should be
prevented.

Additionally, a check for the case where agno is greater or same than
MAXAG should be added in diAlloc() to prevent out-of-bounds.

Reported-by: Jeongjun Park <aha310510@gmail.com>
Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Jeongjun Park <aha310510@gmail.com>
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>