jfs: avoid undefined behavior from left-shifting by 32 bits

Shifting a 32-bit int by 32 bits is undefined behavior in C, and
results in different behavior on different architectures (e.g., x86
and PowerPC).  diAlloc() in fs/jfs/jfs_imap.c computes a mask using
0xffffffffu<<(32-bitno), which can left-shift by 32 bits.  To avoid
unexpected behavior, explicitly check for bitno==0 and use a 0 mask.
Signed-off-by: Nickolai Zeldovich <nickolai@csail.mit.edu>
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>

fs/jfs/jfs_imap.c

@@ -1493,7 +1493,7 @@ int diAlloc(struct inode *pip, bool dir, struct inode *ip)
 		/* mask any prior bits for the starting words of the
 		 * summary map.
 		 */
-		mask = ONES << (EXTSPERSUM - bitno);
+		mask = (bitno == 0) ? 0 : (ONES << (EXTSPERSUM - bitno));
 		inosmap = le32_to_cpu(iagp->inosmap[sword]) | mask;
 		extsmap = le32_to_cpu(iagp->extsmap[sword]) | mask;