selftests: vm: fix shellcheck warnings in run_vmtests.sh

These might not be issues yet, but they make the script more fragile. 
Also by fixing them we give a better example to future readers, who might
copy/paste or otherwise re-use snippets from our script.

- Use "read -r", since we don't ever want read to be interpreting '\'
  characters as escape sequences...
- Quote variables, to deal with spaces properly.
- Use $() instead of the older and harder-to-nest ``.
- Get rid of superfluous "$" prefixes inside arithmetic $(()).

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Axel Rasmussen <[email protected]>
Cc: Shuah Khan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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