MINOR: vars: Move UPDATEONLY flag test to vars_set_ifexist

MINOR: vars: Move UPDATEONLY flag test to vars_set_ifexist

The vars_set_by_name_ifexist function was created to avoid creating too
many variables from a LUA module. This was made thanks to the
VF_UPDATEONLY flags which prevented variable creation in the var_set
function. Since commit 3a4bedccc ("MEDIUM: vars: replace the global name
index with a hash") this limitation was restricted to 'proc' scope
variables only.
This patch simply moves the scope test to the vars_set_by_name_ifexist
function instead of the var_set function.