tests: avoid false failure with spaces in mount point paths

tests: avoid false failure with spaces in mount point paths

* tests/ls/readdir-mountpoint-inode.sh: Quote appropriately
to process mount points with spaces in the path.
Previously items like these would usually be skipped,
though if the path also contained a '-' for example,
that would stat stdin, thus producing a wrong inode
and a false failure.
Fixes http://bugs.gnu.org/17863