6ca33a88 sets upper limit for vacuum_cleanup_index_scale_factor to
DBL_MAX.  DBL_MAX appears to be platform-dependent. That causes
many buildfarm animals to fail, because we check boundaries of
vacuum_cleanup_index_scale_factor in regression tests.

This commit changes upper limit from DBL_MAX to just "large enough"
limit, which was arbitrary selected as 1e10.

Author: Alexander Korotkov
Reported-by: Tom Lane, Darafei Praliaskouski
Discussion: https://postgr.es/m/CAPpHfdvewmr4PcpRjrkstoNn1n2_6dL-iHRB21CCfZ0efZdBTg%40mail.gmail.com
Discussion: https://postgr.es/m/CAC8Q8tLYFOpKNaPS_E7V8KtPdE%3D_TnAn16t%3DA3LuL%3DXjfOO-BQ%40mail.gmail.com