common: fix "utility required warning" with empty utility name

common: fix "utility required warning" with empty utility name

In generic/019, if we hadn't install fio, we will get following output:

 generic/019      [not run]  utility required, skipped this test <- *
 Not run: generic/019
 Passed all 0 tests

When fio is not installed, "$FIO_PROG" is set to blank, and
_require_fio() call _require_command() with none arguments.

This patch fixed all misuse of _require_command(), add 2nd argument
to let _require_command() output right message, and add quotes to
first argument to avoid argument shifting.

Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>