Bugfix: Correct number of blocks reported in mount point

Restic mount points incorrectly reported the 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
