common/rc: ensure modules are loadable in _have_modules()

common/rc: ensure modules are loadable in _have_modules()

The commit e9645877fbf0 ("common: add a helper if a driver is
available") introduced the helper function _have_driver() to check the
driver or module is available no matter whether it is a loadable module
or built-in module. It was assumed that _have_modules() whould check
that specified modules are loadable and not built-in.

However, the function _have_modules() returns true even if the specified
modules are built-in and not loadable. This causes failures of some test
cases on test system with built-in modules such as nbd/004. It also
means that _have_modules() and _have_driver() have same functionality.

To avoid the unexpected failures, fix _have_modules() to return false
when the specified modules are built-in. Check if loadable module file
exists by searching the module file path. If the module file does not
exist, return false. Also add comments to describe the difference
between _have_driver() and _have_modules().

Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>