Go back to $PWD in fsstress.sh if compiling from source.

Although fsstress was being called with a static path the directory
it was writing to was in the current directory so doing a cd to the
source directory that is made in /tmp and then removing it later
caused it to be unable to write the files in a non-existent dir.

This change gets the current path first and cd's back into it after
it is done compiling fsstress.

Issue #6479.

Signed-off-by: Sandon Van Ness <sandon@inktank.com>
Reviewed-by: Alfredo Deza <alfredo.deza@inktank.com>