Bugfix: Correct number of blocks reported in mount point

Restic mount points reported an incorrect number of 512-byte (POSIX standard)
blocks for files and links due to a rounding bug. In particular, empty files
were reported as taking one block instead of zero.

The rounding is now fixed: the number of blocks reported is the file size
(or link target size) divided by 512 and rounded up to a whole number.

https://github.com/restic/restic/issues/4239
https://github.com/restic/restic/pull/4240
