Branch b1_4

Branch b1_4
OSTs running 2.4 kernels but with extents enabled might rarely trip an
assertion in the ext3 JBD (journaling) layer.

The b_committed_data struct is protected by the big kernel lock
in 2.4 kernels, serializing journal_commit_transaction() and
ext3_get_block_handle->ext3_new_block->find_next_usable_block()
access to this struct.  In 2.6 kernels there is finer grained
locking to improve SMP performance of the JBD layer.

b=6198
r=alex (original patch)