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>