libmount: don't report fsconfig errors with "nofail"

libmount: don't report fsconfig errors with "nofail"

The new kernel API returns EINVAL on FSCONFIG_CMD_CREATE if the mount
source is inaccessible. We do not want to report this as an error when
the "nofail" mount option is specified.

Note that EINVAL may also be returned by other fsconfig() settings, so
we need to check whether a source is specified and whether it is really
inaccessible. This is just a heuristic (as with the old mount(2)).

Fixes: https://github.com/util-linux/util-linux/issues/3741
Signed-off-by: Karel Zak <kzak@redhat.com>