FileJournal::wrap_read_bl: adjust pos before returning

Otherwise, we may feed an offset past the end of the journal to
check_header in read_entry and incorrectly determine that the entry is
corrupt.

Fixes: 4296
Backport: bobtail
Backport: argonaut
Reviewed-by: Sage Weil <sage@inktank.com>
Signed-off-by: Samuel Just <sam.just@inktank.com>
(cherry picked from commit 5d54ab154ca790688a6a1a2ad5f869c17a23980a)
rgw: don't iterate through all objects when in namespace

Fixes: #4363
Backport: argonaut, bobtail
When listing objects in namespace don't iterate through all the
objects, only go though the ones that starts with the namespace
prefix

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
(cherry picked from commit 6669e73fa50e3908ec825ee030c31a6dbede6ac0)