osd/PrimaryLogPG: fix the extent length error of the sync read

osd/PrimaryLogPG: fix the extent length error of the sync read

The extent.length of the sync read is changed to 0, when
the read is failed. This makes the retry of the read with
a offset of 0 to read the total object.
Let the extent.length as it is, if the return code is -EAGAIN,
which will make the read try again later with the correct
extent.length.

Fixes: https://tracker.ceph.com/issues/37680
Signed-off-by: Xiaofei Cui <cuixiaofei@sangfor.com.cn>
(cherry picked from commit fd5d2d6d6eb39151d6d96105e83bfd39749d79d5)