small_io_bench_fs.cc: check return value of FileStore::mkfs/mount()

CID 743398 (#1 of 1): Unchecked return value (CHECKED_RETURN)
  check_return: Calling function "FileStore::mount()" without
   checking return value (as is done elsewhere 4 out of 5 times).
  unchecked_value: No check of the return value of "fs.FileStore::mount()"

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>