filter_qualify: move memory allocation from parse_inject_expression

filter_qualify: move memory allocation from parse_inject_expression

* filter_qualify.c (parse_inject_expression): Replace "s" and "buf"
arguments with "str" argument, use it instead of "s" and "*buf".
(qualify_inject_common): Rename "buf" to "copy", initialize it to a copy
of "str", pass "copy" to parse_inject_expression instead of "str" and
"buf".