Use bio(s) to read in the journal sequentially in large chunks and
locate the head of the journal.

This version addresses the issues Christoph pointed out w.r.t error handling
and using deprecated API.
Signed-off-by: NAbhi Das <adas@redhat.com>
Signed-off-by: NAndreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: NBob Peterson <rpeterso@redhat.com>
Cc: Christoph Hellwig <hch@infradead.org>