Merge pull request #6332 from dx9/wip-12406-test_librdb_fsx

test/librbd/fsx: Use c++11 std::mt19937 generator instead of random_r()

Improve portability - random_r() is GNU-specific.

Reviewed-by: Ilya Dryomov <idryomov@gmail.com>

diff --cc src/test/librbd/fsx.cc
Simple merge