AP_Filesystem: lseek limits the max filesize to ~2Gbyte, limits terrain_spacing to 5 meters

[EosBandi]

Bug report

Apparently the SD card filesystem (nor other filesystems) cannot handle files larger than 2Gbyte. In theory the exFAT filesystem on the SD card can handle larger files, but the lseek function in AP_Filesystem limits the offset to an int32_t. It is causing issues with terrain .dat files larger than 2GByte, the offset calculation simply overflows and get totally wrong blocks from the file or writing block at random positions, corrupting the .dat file.

Version
4.6.0-dev and all previous versions

Platform
[ X] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[ ] Rover
[ ] Submarine