use last mount point in ext to correct absolute path symbolic links

[joachimmetz]

Symbolic links can contain an absolute path. If the file system was mounted this might result in an unresolvable link. Use last mount point in ext to try to correct the path.

• check if pytsk exposes sb->last_mnt https://github.com/sleuthkit/sleuthkit/blob/4224e2edbe9d46ccf5f726f417dd0a39f678658c/tsk/fs/ffs.c#L1327
• change pyfsextfs to expose last mount point https://github.com/libyal/libfsext/issues/2
• use mount path spec https://github.com/log2timeline/dfvfs/issues/606?