os/bluestore/BlueStore: Fix bug when calc offset & end whether locate in the a extent.

os/bluestore/BlueStore: Fix bug when calc offset & end whether locate in the a extent.

Suppose: bluestore_overlay_max_length == bluestore_min_alloc_size
The orignal code which calc content of written whether locate in a
extent:
(offset / min_alloc_size) == (offset + length) /min_alloc_size
This will make the case which offset=0 & length =min_alloc_size locate
in the different extent.
In fact, this content is in the same extent.

Change end = offset + length - 1 make work.

Fixes: #14954
Signed-off-by: Jianpeng Ma <jianpeng.ma@intel.com>