lib/vfscore: Fix write const path in automount

lib/vfscore: Fix write const path in automount

Previously vfscore_mount_volume would pass a const path to
vfscore_ukopt_mkmp, which expects a mutable path, and indeed does modify
it in-place during execution. This is wrong and rightfully triggers a
compiler warning.
This change fixes this by having mkmp allocate a temporary writable path.

Checkpatch-Ignore: STRCPY
Signed-off-by: Andrei Tatar <andrei@unikraft.io>
Reviewed-by: Michalis Pappas <michalis@unikraft.io>
Approved-by: Michalis Pappas <michalis@unikraft.io>
GitHub-Closes: #1636