Patch from "Stephen C. Tweedie" <sct@redhat.com>

Fix "h_buffer_credits<0" assert failure during truncate.

The bug occurs when the "i_blocks" count in the file's inode overflows
past 2^31.  That works fine most of the time, because i_blocks is an
unsigned long, and should go up to 2^32; but there's a place in truncate
where ext3 calculates the size of the next transaction chunk for the
delete, and that mistakenly uses a signed long instead.  Because the
huge i_blocks gets cast to a negative value, ext3 does not reserve
enough credits for the transaction and the above error results.

This is usually only possible on filesystems corrupted for other
reasons, but it is reproducible if you create a single, non-sparse file
larger than 1TB on ext3 and then try to delete it.