libext2fs: allow the default journal size to go as large as a gigabyte

libext2fs: allow the default journal size to go as large as a gigabyte

Recent research has shown that for a metadata-heavy workload, a 128 MB
is journal be a bottleneck on HDD's, and that the optimal journal size
is proportional to number of unique metadata blocks that can be
modified (and written into the journal) in a 30 second window.  One
gigabyte should be sufficient for most workloads, which will be used
for file systems larger than 128 gigabytes.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>