AP_Filesystem: littlefs: fix lseek

lseek must return the current file position. Previously, the littlefs
version always returned 0, which broke terrain I/O as it checks that the
position returned is the one it seeked to. Fix to return the current
position, which is correctly returned from littlefs.

This problem was originally and incorrectly diagnosed as an issue with
littlefs seeking past the end of the file, but this functionality works
fine and fixing the incorrect return completely fixes terrain.

Terrain functionality was verified using `TERRAIN_DEBUG` on
KakuteH7Mini-Nand running sim on HW. Terrain data is correctly
downloaded from the GCS and loaded from the filesystem after reboot.

Author: tpwrules

libraries/AP_Filesystem/AP_Filesystem_FlashMemory_LittleFS.cpp

@@ -230,14 +230,13 @@ int32_t AP_Filesystem_FlashMemory_LittleFS::lseek(int fd, int32_t position, int
         break;
     }
 
-    lfs_soff_t size = lfs_file_size(&fs, &(fp->file));
-    // emulate SEEK_SET past the end by truncating and filling with zeros
-    if (position > size && whence == SEEK_SET) {
-        LFS_CHECK(lfs_file_truncate(&fs, &(fp->file), position));
+    lfs_soff_t pos = lfs_file_seek(&fs, &(fp->file), position, lfs_whence);
+    if (pos < 0) {
+        errno = errno_from_lfs_error(pos);
+        return -1;
     }
 
-    LFS_CHECK(lfs_file_seek(&fs, &(fp->file), position, lfs_whence));
-    return 0;
+    return pos;
 }
 
 int AP_Filesystem_FlashMemory_LittleFS::stat(const char *name, struct stat *buf)