Patch from: Hugh Dickins <hugh@veritas.com>

For almost a year (since 2.5.4) ext2_new_block has tended to set err 0
instead of -ENOSPC or -EIO.  This manifested variously (typically depends on
what's stale in ext2_get_block's chain[4] array): sometimes __brelse free
free buffer backtraces, sometimes release_pages oops, usually
generic_make_request beyond end of device messages, followed by further ext2
errors.

[Insert lecture on dangers of using goto for unwind :-]