Easiest/safest would be to reboot, then extend the /dev/mapper/mpath0p8 filesystem with "resize2fs /dev/mapper/mpath0p8" assuming you are using ext2/3/4. Also assuming mpath0p8 is at the end of the disk.
If you can't reboot, run "multipath -l" to find which sdX devices make up mpath0, then run
echo 1 > /sys/block/sdX/device/rescan
for each member of mpath0. That 'should' make the Linux kernel redetect the disk parameters. Then you should be able to resize2fs mpath0p8.