Insufficient free space: X extents needed, but only Y available
I think thare is some roundings at work that cause your lvextend to fail.
Try this: lvextend /dev/vg_webserver/lv_root -l+100%FREE
The above command will ask the system to allocate all free space to the selected logical volume, without asking for a specific size.
You can run grab all free spaces: lvextend /dev/vg_webserver/lv_root -l+100%FREE
Following that you need to run:
In case of xfs: xfs_growfs <partition>
For ext4: resize2fs <partition>