[CHKDSK][VFATLIB] Chkdsk, fix writes to wrong disk offsets (#8840)

CORE-20546

When an alignment is needed and a new read is required,
 make sure we seek back to the read position for our write.

sdk/lib/fslib/vfatlib/check/io.c

@@ -374,6 +374,9 @@ void fs_write(off_t pos, int size, void *data)
             memcpy((char *)scratch + seek_delta, data, size);
         }
 
+        /* Seek back to the beginning of our read/write */ 
+        if (lseek(fd, seekpos_aligned, 0) != seekpos_aligned) pdie("Seek to %lld",seekpos_aligned);
+
         /* Write it back */
         if ((did = write(fd, scratch, readsize_aligned)) == (int)readsize_aligned)
         {