jfs: fix regression preventing coalescing of extents

Commit fec1878f caused a regression in
which contiguous blocks being allocated to the end of an extent were
getting a new extent created.  This typically results in files entirely
made up of 1-block extents even though the blocks are contiguous on
disk.

Apparently grub doesn't handle a jfs file being fragmented into too many
extents, since it refuses to boot a kernel from jfs that was created by
the 2.6.30 kernel.

Signed-off-by: Dave Kleikamp <shaggy@linux.vnet.ibm.com>
Reported-by: Alex <alevkovich@tut.by>